Beam shaping acoustic signal travel time flow meter

ABSTRACT

A method and corresponding device are provided for determining a flow speed in a fluid conduit. The fluid conduit is provided with first, second and third ultrasonic transducers, wherein respective connection lines between transducers extend outside of a symmetry axis of the fluid conduit. First and second measuring signals are applied to the first ultrasonic transducer and received at the second and the third ultrasonic transducer, respectively. The measuring signals comprise a respective reversed signal portion with respect to time of a response signal. Respective first and second response signals are measured and the flow speed is derived from at least one of the first and second response signals.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/843,552, filed Dec. 15, 2017, which is a continuation under 35 U.S.C.§ 120 of International Application No. PCT/IB2016/050218, filed Jan. 18,2016, the contents of each are hereby incorporated by reference in theirentirety.

FIELD OF THE INVENTION

The current application relates to flow meters, and in particular toultrasound travel time flow meters.

BACKGROUND

Various types of flow meters are currently in use for measuring a volumeflow of a fluid, such as a liquid or a gas, through a pipe. Ultrasonicflow meters are either Doppler flow meters, which make use of theacoustic Doppler effect, or travel time flow meters, sometimes alsocalled transmission flow meters, which make use of a propagation timedifference caused by the relative motion of source and medium. Thetravel time is also referred to as time of flight or transit time.

An ultrasonic travel time flow meter evaluates the difference ofpropagation time of ultrasonic pulses propagating in and against flowdirection. Ultrasonic flow meters are provided as in-line flow meters,also known as intrusive or wetted flow meters, or as clamp-on flowmeters, also known as non-intrusive flow meters. Other forms of flowmeters include Venturi channels, overflow sills, radar flow meters,Coriolis flow meters, differential pressure flow meters, magneticinductive flow meters, and other types of flow meters.

When there are irregular flow profiles or open channels, more than onepropagation path may be necessary to determine the average flow speed.Among others, multipath procedures are described in hydrometricstandards such as IEC 41 or EN ISO 6416. As a further application,ultrasonic flow meters are also used to measure flow profiles, forexample with an acoustic Doppler current profiler (ADCP). The ADCP isalso suitable for measuring water velocity and discharge in rivers andopen waters.

SUMMARY OF INVENTION

It is an object of the present specification to provide an improvedtransit time flow meter and a corresponding computer-implemented methodfor measuring an average flow speed or a flow profile of a fluid ingeneral, and in particular for liquids such as water or for gases.

In a flow measurement device according to the present specification,sound transducers, e.g. in the form of piezoelectric elements, alsoknown as piezoelectric transducers, are used to generate and to receivea test signal and a measuring signal.

Alternative sound transmitters comprise lasers that excite a metalmembrane or other light absorbing surface to vibrations, or coil drivenloudspeakers. One can also produce pressure waves in other ways. Thereceiver side can also be represented by other means that are differentfrom piezoelectric transducers, but detect ultrasonic waves.

Although the term “piezoelectric transducer” is used often in thepresent description, it stands also for other sound wave transducersthat produce or detect ultrasonic waves.

A measuring signal according to the present specification can bemodelled by a matched filter. If a sharply peaked impulse is used as aprobe or test signal, the received signal at the transducer is theimpulse response of a conduit or channel of the fluid. According to thepresent application, an inverted version of the impulse response withrespect to time is sent back through the same channel as a measuringsignal, either in the reverse direction or in the same direction. Thisresults in a signal with a peak at the origin, where the original sourcewas, or in a signal with a peak at the original receiver, respectively.

The inversion with respect to time can be achieved in several ways. Ifanalogue means are used for recording the response signal, one couldplay the recorded response signal in a reverse mode. If digital meansare used for recording samples of the response signal, then the order ofthe recorded samples is reversed in order to obtain the inverted signal.This can be achieved by inverting the values of the time stamps of eachrecorded sample, by multiplying the respective time value with (−1). Ifplayed according to an ascending order of the time stamp values, therecorded samples are played in a reverse order. In other words, theinverted response signal is the recorded response signal, but playedbackwards.

An ultrasonic flow meter according to the present specification providesa focusing property by using the above mentioned inverted signal, or asimilarly shaped signal, for an ultrasonic flow meter to form a responsesignal, which is both concentrated in space and time. This in turn leadsto a higher amplitude at a receiving piezoelectric element and a bettersignal to noise ratio.

With an ultrasonic flow meter according to the present specification,focusing and beam forming properties can be obtained under very generalconditions. For example, a focusing property is obtained even when onlyone ultrasound transmitter is excited and even when the inverted signalis reduced to a signal that is only coarsely digitized in the amplituderange, if the time resolution of the inverted signal is sufficient.Furthermore, a flow meter according to the present specification can beused with clamp-on transducers, which are easy to position on a pipe anddo not require modifications of the pipe.

In an ultrasonic flow meter according to the present specification,technical features that ensure a good coupling and directionality ofclamp-on transducers and to reduce scattering may not be necessary or,on the contrary, it may even improve the beam forming characteristic toomit them. In order to provide an increased scattering, a couplingmaterial may be selected that is adapted to a refractive index of theliquid or transducers and transducer couplings may be used, whichprovide more shear waves.

Preferentially, the frequency of sound waves that are used in a flowmeter according to the specification is between >20 kHz and 2 MHz, whichcorresponds to an oscillation period of 0.5 microseconds (μs) but it mayeven be as high as 800 MHz. In many cases, ultrasonic flow metersoperate far above the hearing threshold with frequencies of severalhundred kHz or higher. The frequency of transit time ultrasonic flowmeters is typically in the kHz or in the MHz range.

According to one aspect, the current specification discloses a computerimplemented method for determining a flow speed of a fluid in a fluidconduit or channel, in particular in a pipe or tube, using atransmission time ultrasonic flow meter. In a preferred embodiment,“computer implemented” refers to an execution on small scale electroniccomponents such as microprocessors, ASICs, FPGAs and the like, which canbe used in portable or in compact stationary digital signal processingdevices, which are generally of a smaller size than workstations ormainframe computers and which can be placed at a required location alonga fluid pipe.

In the following, the terms “channel”, “conduit”, “passage”, etc. areused as synonyms. The subject matter of the application can be appliedto all types of conduits for fluids independent of their respectiveshape and independent of whether they are open or closed. The subjectmatter of the application can also be applied to all types of fluids orgases, whether they are gases or liquids, or a mixture of both.

Throughout the application, the term “computer” is often used. Althougha computer includes devices such as a laptop or a desktop computer, thesignal transmission and receiving can also be done by microcontrollers,ASICs, FPGAs, etc.

Furthermore, a connection line between the transducers may be offsetwith respect to a center of the fluid conduit in order to obtain a flowspeed in a predetermined layer and there may be more than one pair oftransducers. Furthermore, the measuring signal may be provided by morethan one transducer and/or the response signal to the measuring signalmay be measured by more than one transducer.

A signal energy E of a signal s(t) in a time interval may be defined interms of the expression E=∫_(T1) ^(T2)dt|s(t)|² or its discrete versionE=Σ_(i=m) ^(n)|s(i)|², wherein the time interval is given by [T1, T2] or[m*Δt, n*Δt], respectively.

The leading portion of the measuring signal may contribute significantlyto the production of a signal, which is peaked in space and time.

In some specific embodiments, the measuring signal or the responsesignal can be provided by an amplitude-modulated oscillating signal,which is digitized with respect to amplitude, e.g. with a resolutionbetween 1 and 12 bit. This may provide benefits in terms of computationvelocity and memory space and can even lead to an increased signal peak.In particular, the data shown in the Figures of the presentspecification have been obtained with 12 bit resolution, except forFIGS. 30-35, which have been obtained with a coarser resolution than 12bit.

According to a further embodiment, the measuring signal that is appliedto a transducer can comprise an oscillating signal that is modulatedaccording to a 0-1 modulation providing either a predetermined amplitudeor no amplitude, or, in other words a zero amplitude.

According to a further aspect, a device for measuring a flow speed in atravel time ultrasonic flow meter is disclosed. The device comprises afirst connector for connecting a first piezoelectric element, a secondconnector for connecting a second piezoelectric element, an optionaldigital to analog converter (DAC), which is connected to the firstconnector and an optional analog to digital converter (ADC), which isconnected to the second connector.

Furthermore, the device comprises a computer readable memory, anelectronic timer or oscillator, a transmitting unit for sending animpulse signal to the first connector and a receiving unit for receivinga response signal to the impulse signal from the second connector.

The terms velocity of flow, flow velocity and flow speed are used assynonyms in the present application.

While the device can be provided as an analog device without A/D and D/Aconverters and without a computer readable memory unit, it is alsopossible to provide the device or parts of it with a digital computersystem.

In particular, the various signal processing units, such as thevelocity-processing unit, the selection unit and the inverting unit maybe provided entirely or partially by an application specific electroniccomponent or by a program memory with a computer readable instructionset. Similarly, the measuring signal generator and an impulse signalgenerator of the transmitting unit may be provided entirely or partiallyby an application specific electronic component, which may comprise acomputer readable instruction set.

According to a further embodiment, the device comprises a direct digitalsignal synthesizer (DDS) that comprises the abovementioned ADC. The DDScomprises a frequency control register, a reference oscillator, anumerically controlled oscillator and a reconstruction low pass filter.Furthermore, the ADC is connectable to the first and to the secondconnector over the reconstruction low pass filter.

Furthermore, the current specification discloses a flow measurementdevice with a first piezoelectric transducer that is connected to thefirst connector, and with a second ultrasonic transducer, such aspiezoelectric transducer, that is connected to the second connector. Inparticular, the ultrasonic transducers, such as piezoelectrictransducers may be provided with attachment regions, such as a clampingmechanism for attaching them to a pipe.

Furthermore, the current specification discloses a flow measurementdevice with a pipe portion. The first ultrasonic transducer, such aspiezoelectric transducer is mounted to the pipe portion at a firstlocation and the second ultrasonic transducer, such as piezoelectrictransducer is mounted to the pipe portion at a second location. Inparticular, the transducers may be clamped to the pipe portion.Providing the device with a pipe portion may provide benefits when thedevice is pre-calibrated with respect to the pipe portion.

The device can be made compact and portable. A portable device accordingto the present specification, which is equipped with surface mountabletransducers, such as clamp-on transducers, can be used to check a pipeon any accessible location. In general, the device may be stationary orportable. Preferentially, the device is sufficiently compact to beplaced at a required location and sufficiently protected againstenvironmental conditions, such as humidity, heat and corrosivesubstances.

Moreover, the current specification discloses a computer readable codefor executing a flow measurement method according to the presentspecification, a computer readable memory comprising the computerreadable code and an application specific electronic component, which isoperable to execute the method steps of a method according to thecurrent specification.

In particular, the application specific electronic component may beprovided by an electronic component comprising the abovementionedcomputer readable memory, such as an EPROM, an EEPROM a flash memory orthe like. According to other embodiments, the application specificelectronic component is provided by a component with a hard-wired orwith a configurable circuitry such as an application specific integratedcircuit (ASIC) or a field programmable gate array (FPGA).

In a further embodiment, an application specific electronic componentaccording to the current specification is provided by a plurality ofinterconnected electronic components, for example by an FPGA, which isconnected to a suitably programmed EPROM in a multi-die arrangement.Further examples of an application specific electronic component areprogrammable integrated circuits such as programmable logic arrays(PLAs) and complex programmable logic devices (CPLDs).

It is helpful to determine whether an off-the-shelf test device ismeasuring a flow speed of a fluid in a fluid conduit according topresent application. To this purpose, one provides the fluid conduitwith a fluid that has a pre-determined velocity with respect to thefluid conduit. A test impulse signal is applied to a first ultrasonictransducer, such as piezoelectric transducer of the test device, thefirst piezoelectric transducer being mounted to the fluid conduit at afirst location, followed by receiving a test response signal of the testimpulse signal at a second piezoelectric transducer of the test device,the second ultrasonic transducer, such as piezoelectric transducer beingmounted to the fluid conduit at a second location.

Furthermore, the present specification discloses a computer-implementedmethod for determining a flow speed of a fluid in a fluid conduit usingpre-determined measurement signals in an arrangement with three or moreultrasonic transducers. The pre-determined signals comprise a firstmeasuring signal and a second measuring signal.

A fluid conduit is provided with a fluid that has a predeterminedvelocity with respect to the fluid conduit. Furthermore, the fluidconduit is provided with a first ultrasonic transducer, a secondultrasonic transducer and a third ultrasonic transducer. In particular,the second transducer and the third transducer can be placed at adistance with respect to the first transducer and with respect to alongitudinal direction of the conduit.

The transducers are arranged such that respective connection linesbetween the first ultrasonic transducer, the second ultrasonictransducer and the third ultrasonic transducer extend outside of asymmetry axis of the fluid conduit. In particular, the connection linecan be offset by 5% or more or by 10% or more with respect to a meandiameter of the conduit or with respect to a mean radius of the conduit.

For example, a mean radius of the conduit can be defined with respect toa reference point on the symmetry axis as

$\frac{1}{2\; \pi}{\int_{0}^{2\; \pi}{{r}\ d\; \phi \mspace{11mu} {or}\mspace{14mu} {as}\mspace{11mu} \sqrt{\frac{1}{2\; \pi}{\int_{0}^{2\; \pi}{r^{2}\ d\; \phi}}}\mspace{14mu} {{etc}.}}}$

A first pre-determined measuring signal is applied to the firstultrasonic transducer, and a first response signal of the firstpre-determined measuring signal received at the second ultrasonictransducer is measured, for example by detecting a voltage emitted bythe second ultrasonic transducer.

Likewise, a second pre-determined measuring signal is applied to thefirst ultrasonic transducer and a second response signal of the secondpre-determined measuring signal received at the third ultrasonictransducer is measured.

The first pre-determined measuring signal and the second pre-determinedmeasuring signal respectively comprise a reversed signal portion withrespect to time of a response signal of a corresponding impulse signalor of a signal derived therefrom.

In particular, the respective measurement signal can be generated froman impulse signal which is sent between the same pair of transducers asthe measurement signal. The generation of the measurement signal can becarried out by an actual measurement, by a simulation or by acombination of both.

The measurement signal can be sent in the same direction or in thereverse direction as the impulse signal from which it is generated. Inparticular, if the fluid is moving with respect to the conduit duringthe calibration process by which the measurement signal is generatedfrom the impulse signal, it can be advantageous for reasons of stabilityto send the measurement signal in the same direction as the impulsesignal.

In other words, in a measurement phase the sequence of sendingtransducer and receiving transducer can be the same as in a precedingcalibration phase for generating the measurement signal or,alternatively, it can be reversed by using the previously sendingtransducer as a receiving transducer and the previously receivingtransducer as a sending transducer.

In general, the first measuring signal is adapted to a transmissionchannel or path that is different from a transmission channel of thesecond measuring signal. Thereby, the first measuring signal and thesecond measuring signal are in general different from each other.Furthermore, a measurement signal obtained by sending an impulse signalfrom a first transducer to a second transducer is in general differentfrom a measurement signal obtained by sending the impulse signal in thereverse direction from the second transducer to the first transducer.

In general the signal propagation of the pressure signal between thesending and the receiving transducer does not only comprise a straightpropagation, but may also comprise one or more reflections at a conduitwall and/or scattering processes within the pipe wall.

In particular, the abovementioned method can be used in a time of flight(TOF) flow measurement. For the TOF measurement, the steps of applyingthe first measuring signal and measuring a corresponding response signaland of applying the second measuring signal and measuring acorresponding response signal are repeated in a reverse direction toobtain corresponding first reverse direction and second reversedirection response signals.

As explained above, performing the measurement “in reverse direction”refers to carrying out the measurement steps such that the roles of therespective transducers are exchanged, or in other words, such that thepreviously sending transducer is used as a receiving transducer and thepreviously receiving transducer is used as a sending transducer.

Thus, if a first measurement comprises sending a measurement signal in aflow direction of the fluid in the sense that the measurement signal hasa velocity component in direction of the fluid flow then thecorresponding measurement “in reverse direction” comprises sending themeasurement signal against the flow direction of the fluid.

The first response signal and the second response signal to therespective first measurement signal and second measurement signal areused to derive one or more flow speeds of the fluid. In particular, thefirst response signal can be used to determine a flow speed in a fluidlayer which comprises the connection line between the first transducerand the second transducer, and the second response signal can be used todetermine a flow speed in a fluid layer which comprises the connectionline between the first transducer and the third transducer.

According to a further embodiment, which is suitable for a time offlight measurement a first predetermined reverse direction measuringsignal is applied to the second ultrasonic transducer, and a firstreverse direction response signal of the first reverse directionmeasuring signal at the second ultrasonic transducer is measured.

Similarly, a second reverse direction measuring signal is applied to thethird ultrasonic transducer and a second reverse direction responsesignal of the second measuring signal received at the first ultrasonictransducer is measured.

The first reverse direction measuring signal and the second reversedirection measuring signal respectively comprise a reversed signalportion with respect to time of a response signal of a correspondingimpulse signal or of a signal derived therefrom. ‘Corresponding impulsesignal’ refers to an impulse signal that is sent between the same pairof transducers as the corresponding measurement signal.

A flow speed of the fluid is derived from at least one of the firstresponse signal, the first reverse direction response signal, the secondresponse signal and the second reverse direction response signal. Inparticular, the first response signal and the first reverse directionresponse signal can be used to derive a flow speed using a time offlight method. Similarly, the second response signal and the secondreverse direction response signal can be used to derive a flow speedusing a time of flight method.

In a further embodiment, which also uses a measurement signal travelingfrom the second transducer to third transducer in the abovementionedarrangement of three transducers, the measurement comprises furthermorethe following steps.

A third measuring signal is applied to the second ultrasonic transducerand a third response signal of the second measuring signal at the thirdultrasonic transducer is measured.

Similar to the abovementioned embodiments, the third predeterminedmeasuring signal comprises a reversed signal portion with respect totime of a response signal of a corresponding impulse signal or of asignal derived therefrom.

At least one flow speed of the fluid is derived from the third responsesignal. For the purpose of determining the at least one flow speed, thefirst response signal, the first reverse direction response signal, thesecond response signal and the second reverse direction response signalmay be used as well.

In a further embodiment, which is suitable for determining a flow speedin fluid layer between the second transducer and the third transducerusing a time of flight method, the method comprises furthermore thefollowing steps.

A third reverse direction measuring signal is applied to the thirdultrasonic transducer and a third reverse direction response signal ofthe third reverse direction measuring signal received at the secondultrasonic transducer is measured.

Similarly to the abovementioned embodiments, the third reverse directionmeasuring signal comprises a reversed signal portion with respect totime of a response signal of a corresponding impulse signal or of asignal derived therefrom.

At least one flow speed of the fluid from is derived from the thirdresponse signal and the third reverse direction response signal. For thepurpose of determining the at least one flow speed, the first responsesignal, the first reverse direction response signal, the second responsesignal and the second reverse direction response signal may be used aswell.

According to a further computer implemented method, which is suitablefor determining a flow speed in a fluid conduit with an arrangement ofat least two clamp-on transducers, the fluid conduit is provided with afluid that has a predetermined velocity with respect to the fluidconduit.

Furthermore, the fluid conduit is provided with a first ultrasonicclamp-on transducer and a second ultrasonic clamp-on transducer.Preferentially, the second ultrasonic clamp-on transducer is offset withrespect to the first ultrasonic clamp-on transducer in a longitudinaldirection of the conduit.

The clamp-on transducers are arranged such that a straight connectionline between the first ultrasonic clamp-on transducer and the secondultrasonic clamp-on transducer extends outside of a symmetry axis of thefluid conduit. In particular, the connection line may be offset withrespect to the symmetry axis by 5% or more, or by 10% or more relativeto a mean diameter or relative to a mean radius of the conduit.

A predetermined measuring signal is applied to the first ultrasonicclamp-on transducer and a response signal of the measuring signalreceived at the second ultrasonic clamp-on transducer is measured.

Similar to the abovementioned embodiment the pre-determined measuringsignal comprises a reversed signal portion with respect to time of aresponse signal of a corresponding impulse signal or of a signal derivedtherefrom. A flow speed of the fluid is derived from the responsesignal.

Similar to the abovementioned embodiments, the measurement phase canalso comprise sending measurement signals in the reverse direction. Inparticular, the method may comprise applying a pre-determined reversedirection measuring signal to the second ultrasonic clamp-on transducerand measuring a reverse direction response signal of the reversedirection measuring signal at the second ultrasonic clamp-on transducer.

Similar to the abovementioned embodiment the reverse direction measuringsignal comprises a reversed signal portion with respect to time of aresponse signal of a corresponding impulse signal or of a signal derivedtherefrom. A flow speed of the fluid is derived from the response signaland from the reverse direction response signal, in particular by using atime of flight method.

The measurement methods for the abovementioned arrangement of at leastthree ultrasonic transducers, which may be provided as wet transducersor as clamp-on transducers, apply in likewise manner also tocorresponding arrangements of clamp-on transducers.

Clamp-on transducers may provide particular advantages in the context ofconcentrating an acoustic signal at a specific location on the conduit,which is also referred to as “beam shaping”. By making use of aninteraction with the conduit and, optionally, also with coupling piecesthe sound waves of the clamp-on transducers can be spread to a widerangle or in more directions as compared to wet transducers. The couplingpieces allow to direct the acoustic waves in accordance with Snell's lawbut also help to generate more modes and scattering.

An inversion with respect to time according to the present specificationcan then be used to generate a measuring signal that adds the varioussignal components traveling along different paths by superposition, andthereby leads to a higher signal amplitude at a specific location of theconduit where a receiving transducer can be placed.

The below mentioned modifications relating to repeated measurements anda digitization step in the generation of the measurement signal can beapplied to all arrangements of transducers according to the presentspecification.

In the abovementioned embodiments, the steps of applying an impulsesignal and receiving a corresponding response signal are repeatedmultiple times and a plurality of response signals is obtained. Inparticular, the repeated measurements may refer to a given combinationof two transducers. The respective measuring signal, such as the firstand the second measurement signal, is then derived from an average ofthe received response signals.

In particular, the derivation of the respective measuring signal in theabovementioned embodiments may comprise digitizing the correspondingresponse signal or a signal derived therefrom with respect to amplitude.According to one embodiment, steps of varying a bit resolution of therespective measuring signal and measuring a response signal to thatmeasuring signal are repeated until a measuring signal is found whichgenerates the response signal with the highest maximum amplitude. Themeasuring signal with the corresponding bit resolution is then selectedas measuring signal.

According to one particular embodiment, the bit-resolution of thedigitized signal is increased for increasing an amplitude of a responsesignal to the respective measuring signal, such as the first and secondmeasuring signal. By way of example, the bit resolution is increased inpre-determined steps, and the bit resolution which produces the responsesignal with the highest amplitude is selected and a correspondingrepresentation of a measurement signal is stored in computer memory.

According to another particular embodiment, the bit-resolution of thedigitized signal is decreased or reduced for increasing an amplitude ofa response signal to the respective measuring signal. By way of example,the bit resolution is decreased in pre-determined steps, the bitresolution which produces the response signal with the highest amplitudeis selected and a corresponding representation of a measurement signalis stored in computer memory.

In particular, the bit resolution of the digitized signal with respectto the amplitude can be chosen as a low bit resolution. For example, thelow resolution may be between a 1 bit and an 8 bit resolution or it maybe between a 1 bit resolution and a 64 bit resolution.

According to one specific embodiment, at least one of the responsesignals to the measurement signals is processed for determining a changein the wall thickness of the conduit or for determining materialcharacteristics of the conduit walls by determining longitudinal andtransversal sound wave characteristics. For example, the transverse andlongitudinal waves characteristics may be derived from correspondingportions of the receiving or response signal, which corresponds todifferent times of arrival of the acoustic waves.

According to a further embodiment, the method comprises a priorcalibration, in which the respective measuring signals are generatedfrom the response signal to an impulse signal. The calibration may becarried out in a factory setting or also during operation of the method.The below mentioned calibration can be applied in likewise manner to allcombinations of pairs of two transducers and the calibration may becarried out in one direction only with respect to a pair of transducersor in both directions with respect to the pair of transducers. In theformer case, one measurement signal is obtained for the pair oftransducers and in the latter case, two measurement signals are obtainedfor the pair of transducers.

During the calibration phase, the fluid conduit is provided with a fluidwhich is at rest relative to the fluid conduit or which is moving with apredetermined velocity relative to the fluid conduit.

A first impulse signal is applied to the second ultrasonic transducer,and a first response signal of the first impulse signal is received atthe first ultrasonic transducer.

Similarly, a second impulse signal is applied to the third ultrasonictransducer and a second response signal of the at least one impulsesignal is received at the first ultrasonic transducer.

The first measuring signal is derived from the first response signal andthe second measuring signal is derived from the second response signal.

The derivation of the respective first and second measuring signalscomprises selecting a signal portion of the respective first and secondresponse signals or of a signal derived therefrom and reversing thesignal portion with respect to time.

In other words, a portion of the first response signal is selected andis inverted or reversed with respect to time and the first measurementsignal is generated using the inverted signal portion. Similarly, aportion of the second response signal is selected and is inverted orreversed with respect to time and the second measurement signal isgenerated using the inverted signal portion.

The first pre-determined measuring signal and the second pre-determinedmeasuring signal for later use. As mentioned above, the same calibrationprocess can be used for every combination of two transducers.

In general, the calibration is carried out in the reverse direction aswell to avoid or compensate for instabilities. Depending on whether thecalibration is performed under zero flow or non-zero flow conditions itcan be advantageous to provide the calibration in both directions and touse each of the two generated measurement signals in either direction.

In other words, if during the measurement process a first transducer isthe sending transducer and a second transducer is the receivingtransducer then the measurement signal may have been generated bysending an impulse signal from the first transducer to the secondtransducer or by sending the impulse signal from the second transducerto the first transducer.

A similar calibration process can be carried out for every pair of twotransducers. In particular, the calibration process can be carried outin a similar manner for every pair of transducers of the abovementionedthree-transducer arrangement comprising a first, second and thirdtransducer.

In the specific case of an arrangement of two or more clamp-ontransducers, a calibration can be carried out by the following steps. Afluid conduit is provided with a fluid. The fluid has a predeterminedvelocity with respect to the fluid conduit in particular.

An impulse signal is provided to the first ultrasonic clamp-ontransducer or to the second ultrasonic clamp-on transducer. Then, aresponse signal of the impulse signal is received at the other one ofthe two ultrasonic transducers and the measuring signal is derived fromthe response signal. Similarly, the impulse signal can be provided atthe second ultrasonic clamp-on transducer and the response signal can bereceived at the first ultrasonic clamp-on transducer.

Herein, the derivation of the measuring signal comprising selecting asignal portion of the respective response signal or of a signal derivedtherefrom and reversing the signal portion with respect to time. Themeasuring signal is stored for later use during a measuring process, inparticular for determining a flow velocity of the fluid.

An impulse signal according to the present specification may refer to asingle impulse signal. In general, an impulse signal refers to a signal,which has a signal energy that is concentrated over a short period oftime. In a specific embodiment, the impulse signal extends over only afew oscillation periods of a carrier, such as 10-20 oscillation periodsor less.

In particular, an envelope of the impulse signal may have a rectangularshape, but other shapes are possible as well. For example, the impulsesignal may correspond to a one-time peak or a single impulse, a shortrectangular burst or to any other signal shape, such as a triangularsaw-tooth shape, a rectangular wave, a chirp, a sine wave or apre-determined noise burst, such as a white noise or a pink noise, whichis also known as 1/f noise. The calibration method works with almost anysignal shape of the impulse signal.

In a further embodiment, a corresponding response signal is sent andreceived multiple times, thereby obtaining a plurality of responsesignals and the respective measuring signal is derived from an averageof the received response signals.

In particular the derivation of the respective measuring signal maycomprise digitizing the corresponding response signal or a signalderived therefrom with respect to amplitude.

As mentioned further above, the bit-resolution of the digitized signalis increased for increasing an amplitude of a response signal to therespective measuring signal. In one particular embodiment, an amplitudeof the response signal to the generated measuring signal is measured ata pre-determined location of the conduit for measurement signalscorresponding to different bit-resolutions. The measurement signal withthe highest amplitude is then selected and stored in memory for lateruse.

A similar procedure can also be provided by decreasing thebit-resolution of the digitized signal until a response signal of themeasurement signal is detected which has a high amplitude and thecorresponding measurement signal is then stored in memory for later use.

In particular, the bit resolution of the digitized signal with respectto the amplitude can be chosen as a low bit resolution such as aresolution between 1 and 10 bit.

Furthermore, the present specification discloses a computer readableprogram code with computer readable instructions for executing one ofthe abovementioned flow measurement methods. Moreover, the presentspecification also discloses a computer readable memory with thecomputer readable program code and an application specific electroniccomponent, which is operable to execute the abovementioned flowmeasurement method.

Furthermore, the present specification discloses a device for measuringa flow speed of a fluid in a conduit having a three transducerarrangement. The device is operative to perform a travel time or time offlight flow measurement.

The device comprises a first connector for connecting a first ultrasonicelement, a second connector for connecting a second ultrasonic element,and a third connector for connecting a third ultrasonic element.

Furthermore, the device comprises a transmitting unit for sendingimpulse signals and for sending measuring signals, a receiving unit forreceiving response signals, and a processing unit. The transmittingunit, the receiving unit and the processing unit are provided forderiving a first measuring signal from a first inverted signal, forderiving a second measuring signal from a second inverted signal and forstoring the first measuring signal and the second measuring signal.

Similar to the abovementioned embodiments, the derivation of theinverted signal comprises reversing a signal portion of a responsesignal of a corresponding impulse signal or of a signal derivedtherefrom with respect to time.

The processing unit, the transmitting unit and the receiving unit areoperative to apply the first pre-determined measuring signal to thefirst connector, and to receive a first response signal of the firstmeasuring signal at the second connector.

Furthermore, the processing unit, the transmitting unit and thereceiving unit are operative to apply a second measuring signal to thefirst connector and to receive a second response signal of the secondmeasuring signal at the third connector, and to derive a flow speed ofthe fluid from at least one of the first response signal and the secondresponse signal.

Furthermore, the processing unit, the transmitting unit and thereceiving unit can be operative to perform any of the other measurementand calibration methods that are described above with respect to a threetransducer arrangement of three transducers, which may be wettransducers or clamp-on transducers.

The application of a signal can comprise in particular retrieving astored signal from computer memory and generate a corresponding electricsignal which is then transmitted to the transducer, in general by meansof a cable. Furthermore, the processing unit is operative to derive aflow speed of the fluid from at least one of the first response signaland the second response signal.

In particular, the connectors, the transmitting unit, the receiving unitand the processing unit can be provided by a travel time ultrasonic flowmeter or a portion thereof, and in particular by a portable travel timeultrasonic flow meter or a portion thereof.

In a further aspect, the present specification discloses a device formeasuring a flow speed of a fluid in a conduit in an arrangement with atleast two clamp-on transducers. In particular,

The device comprises a first connector, a first ultrasonic clamp-ontransducer which is connected to the first connector. Similarly, thedevice comprises a second connector and a second ultrasonic clamp-ontransducer which is connected to the second connector.

Furthermore, the device comprises a portion of a conduit, the firstultrasonic clamp-on transducer being mounted to the conduit portion at afirst location and the second ultrasonic clamp-on transducer beingmounted to the conduit portion at a second location.

The clamp-on transducers are arranged such that respective connectionlines between the first ultrasonic transducer, and the second ultrasonictransducer extend outside of a symmetry axis of the fluid conduit.

Similar to the abovementioned device, the device comprises atransmitting unit for sending impulse signals and for sending measuringsignals, a receiving unit for receiving response signals and aprocessing unit for deriving a measuring signal from an inverted signal.

Similar to the abovementioned embodiments, the inverted signal comprisesa reversed signal portion with respect to time of a response signal of acorresponding impulse signal or of a signal derived therefrom.

The processing unit, the transmitting unit and the receiving unit areoperative to apply the measuring signal to the first connector, toreceive a response signal of the first (pre-determined) measuring signalat the second connector and to derive a flow speed of the fluid from theresponse signal.

Furthermore, the processing unit, the transmitting unit and thereceiving unit of the device can be operative to perform any of theother measurement and calibration methods that are described above withrespect to an arrangement with a first clamp-on transducer and a secondclamp-on transducer.

In a further embodiment, the device comprises a D/A converter, which isconnected to the respective connectors, and an A/D converter, which isconnected to the respective connectors. Furthermore, the devicecomprises a computer readable memory for storing the at least onemeasuring signal.

According to a further embodiment, the device comprises a direct digitalsignal synthesizer, which comprises the ADC, a frequency controlregister, a reference oscillator, a numerically controlled oscillatorand a reconstruction low pass filter. The ADC is connectable to therespective connectors over the reconstruction low pass filter.

According to a further aspect, the current specification discloses acomputer-implemented method for determining whether a given test deviceor device under test is measuring a flow speed of a fluid in a fluidconduit according to the abovementioned measurement method. The testmethod does not provide a mathematical proof that the same method isused but a likelihood, which is sufficient for practical purposes.

According to this method, the fluid conduit is provided with a fluidthat has a pre-determined velocity with respect to the fluid conduit.

The fluid conduit is provided with a first ultrasonic transducer and asecond ultrasonic transducer, which are mounted at respective first andsecond locations.

A test impulse signal is applied to the first ultrasonic transducer ofthe test device, and a test response signal of the test impulse signalis received at the second ultrasonic transducer of the test device.

A first test measuring signal is derived from the first response signal,wherein the derivation of the first measuring signal comprises reversingthe respective first or second response signal, or a portion thereof,with respect to time.

The first test measuring signal is compared with a first measuringsignal that is emitted at a transducer of the test device. It isdetermined that the test device is using a method to determine a flowspeed of a fluid in a fluid conduit according to one of the claims, ifthe first test measuring signal and the first measuring signal aresimilar.

In particular, this method can be performed for every pair oftransducers mentioned in one of the claims and it is detected that thecorresponding method is used if the obtained measuring signals aresimilar for every such pair of transducers.

Specifically, with respect to the method of some of the claims thecorresponding test method can further comprise providing the fluidconduit with a third ultrasonic transducer, applying a test impulsesignal to the first ultrasonic transducer of the test device or to thesecond ultrasonic transducer of the test device, receiving a second testresponse signal of the test impulse signal at the at the thirdultrasonic transducer of the test device, deriving a second testmeasuring signal from the second test response signal, and comparing thesecond test measuring signal with a second measuring signal that isemitted at a transducer of the test device.

It is determined that the test device is using a method to determine aflow speed of a fluid in a fluid conduit according to the claims, if thefirst test measuring signal and the first measuring signal are similar.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the present specification is now explained infurther detail with respect to the following Figures, wherein

FIG. 1 shows a first flow meter arrangement with two piezoelectricelements,

FIG. 2 shows the flow meter arrangement of FIG. 1, one direct signal andtwo scattered signals,

FIG. 3 shows the flow meter arrangement of FIG. 1 when viewed in thedirection of flow,

FIG. 4 shows a second flow meter arrangement with four piezoelectricelements and four direct signals,

FIG. 5 shows the flow meter arrangement of FIG. 4 when viewed in thedirection of flow,

FIG. 6 shows a schematic diagram of a test signal,

FIG. 7 shows a schematic diagram of a test signal response,

FIG. 8 shows a schematic diagram of an inverted signal,

FIG. 9 shows a schematic diagram of a response from the inverted signal,

FIG. 10 shows a first inverted signal in high resolution,

FIG. 11 shows a response of the inverted signal of FIG. 10,

FIG. 12 shows a further inverted signal in high resolution,

FIG. 13 shows a response of the inverted signal of FIG. 12,

FIG. 14 shows a further inverted signal in high resolution,

FIG. 15 shows a response of the inverted signal of FIG. 14,

FIG. 16 shows a further inverted signal in high resolution,

FIG. 17 shows a response of the inverted signal of FIG. 16,

FIG. 18 shows a further inverted signal in high resolution,

FIG. 19 shows a response of the inverted signal of FIG. 18,

FIG. 20 shows a further inverted signal in high resolution,

FIG. 21 shows a response of the inverted signal of FIG. 20,

FIG. 22 shows a further inverted signal in high resolution,

FIG. 23 shows a response of the inverted signal of FIG. 22,

FIG. 24 shows a further inverted signal in high resolution,

FIG. 25 shows a response of the inverted signal of FIG. 24,

FIG. 26 shows a further inverted signal in high resolution,

FIG. 27 shows a response of the inverted signal of FIG. 26,

FIG. 28 shows a further inverted signal in 12-bit resolution,

FIG. 29 shows a response of the signal of FIG. 28,

FIG. 30 shows a further inverted signal in 3-bit resolution,

FIG. 31 shows a response of the signal of FIG. 30,

FIG. 32 shows a further inverted signal in 2-bit resolution,

FIG. 33 shows a response of the signal of FIG. 32,

FIG. 34 shows a further inverted signal in 1-bit resolution,

FIG. 35 shows a response of the signal of FIG. 34,

FIG. 36 shows a short impulse at a piezoelectric element of the flowmeter of FIG. 1,

FIG. 37 shows a signal of a piezoelectric element of the flow meter ofFIG. 1, which is derived from the inverted response of the signal ofFIG. 36,

FIG. 38 shows a response of the signal of FIG. 37,

FIG. 39 shows an upstream and a downstream cross correlation function,

FIG. 40 shows a sectional enlargement of FIG. 39,

FIG. 41 shows a response signal of an inverted signal for a 12-degreemisalignment against an opposite arrangement of piezoelectric elements,

FIG. 42 shows a many-to-one sensor arrangement for a flow measurementaccording to the present specification,

FIG. 43 shows a one-to-many sensor arrangement for a flow measurementaccording to the present specification,

FIG. 44 shows a one-to-one sensor arrangement for a flow measurement ina layer according to the present specification,

FIG. 45 shows a multi-sensor arrangement for flow measure-ment inmultiple layers according to the present specification,

FIG. 46 shows a device for measuring a flow speed according to thepresent specification,

FIG. 47 shows a direct digital synthesizer for use in the device of FIG.46,

FIG. 48 shows a longitudinal cross section of an asymmetric transducerarrangement,

FIG. 49 shows a transverse cross section of the arrangement of FIG. 49,

FIG. 50 shows a one cycle measuring signal of a time of flightmeasurement,

FIG. 51 shows a ten cycle measuring signal of a time of flightmeasurement,

FIG. 52 show a measuring signal that is derived from a time reversedsignal,

FIG. 53 shows a response signal of the signal of FIG. 50, wherein thetransmission channel is provided by the asymmetric arrangement of FIGS.48 and 49,

FIG. 54 shows a response signal of the signal of FIG. 51 for thearrangement of FIGS. 48 and 49,

FIG. 55 shows a response signal of the Signal of FIG. 52 for thearrangement of FIGS. 48 and 49,

FIG. 56 shows a procedure for obtaining measuring signals correspondingto two signal paths in a three transducer arrangement,

FIG. 57 shows a TOF flow measurement using the measuring signalsobtained in the method of FIG. 56,

FIG. 58 shows two different arrangements of two transducers on aconduit,

FIG. 59 shows pressure distributions of measuring signals obtained inthe arrangement of FIG. 58, and

FIG. 60 illustrates an example of determining whether a device undertest uses the same method of flow measurement as a verification device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, details are provided to describe theembodiments of the present specification. It shall be apparent to oneskilled in the art, however, that the embodiments may be practicedwithout such details.

Some parts of the embodiments, which are shown in the Figs., havesimilar parts. The similar parts have the same names or similar partnumbers with a prime symbol or with an alphabetic symbol. Thedescription of such similar parts also applies by reference to othersimilar parts, where appropriate, thereby reducing repetition of textwithout limiting the disclosure.

FIG. 1 shows a first flow meter arrangement 10. In the flow meterarrangement, a first piezoelectric element 11 is placed at an outer wallof a pipe 12, which is also referred as a tube 12. A secondpiezoelectric element 13 is placed at an opposite side of the pipe 12such that a direct line between the piezoelectric element 11 and thedownstream piezoelectric element 13 is oriented at an angle β to thedirection 14 of average flow, which is at the same time also thedirection of the pipe's 12 symmetry axis. The angle β is chosen to beapproximately 45 degrees in the example of FIG. 1 but it may also besteeper, such as for example 60 degrees, or shallower, such as forexample 30 degrees.

A piezoelectric element, such as the piezoelectric elements 11, 13 ofFIG. 1 may in general be operated as an acoustic transmitter and as anacoustic sensor. An acoustic transmitter and an acoustic sensor may beprovided by the same piezoelectric element or by different regions ofthe same piezoelectric element. In this case, a piezoelectric element ortransducer is also referred to as piezoelectric transmitter when it isoperated as transmitter or sound source and it is also referred to asacoustic sensor or receiver when it is operated as acoustic sensor.

When a flow direction is as shown in FIG. 1, the first piezoelectricelement 11 is also referred to as “upstream” piezoelectric element andthe second piezoelectric element 13 is also referred to as “downstream”piezoelectric element. A flow meter according to the presentspecification works for both directions of flow in essentially the sameway and the flow direction of FIG. 1 is only provided by way of example.

FIG. 1 shows a flow of electric signals of FIG. 1 for a configuration inwhich the upstream piezoelectric element 11 is operated as apiezoelectric transducer and the downstream piezoelectric element 13 isoperated as an acoustic sensor. For the purpose of clarity, theapplication works upstream and downstream, i.e. the position of thepiezoelectric elements can be interchanged.

A first computation unit 15 is connected to the upstream piezoelectricelement 11 and a second computation unit 16 is connected to thedownstream piezoelectric element 13. The first computation unit 15comprises a first digital signal processor, a first digital analogconverter (DAC) and a first analog digital converter (ADC). Likewise,the second computation unit 16 comprises a second digital signalprocessor, a second digital analog converter (DAC) and a second analogdigital converter (ADC). The first computation unit 15 is connected tothe second computation unit 16.

The arrangement with two computation units 15, 16 shown in FIG. 1 isonly provided by way of example. Other embodiments may have differentnumbers and arrangements of computation units. For example, there may beonly one central computation unit or there may be two AD/DC convertersand one central computation unit, or there may be two small-scalecomputation units at the transducers and one larger central computationunit.

A computation unit or computation units can be provided bymicrocontrollers or application specific integrated circuits (ASICs), orfield programmable gate arrays (FPGAs), for example. Specifically, thesynthesis of an electrical signal from a stored digital signal may beprovided by a direct digital synthesizer (DDS), which comprises adigital to analog converter (DA, DAC).

A method for generating a measuring signal according to the presentspecification comprises the following steps.

A pre-determined digital test signal is generated by synthesizing anacoustic signal with the digital signal processor of the firstcomputation unit 15. The digital test signal is sent from the firstcomputation unit 15 to the piezoelectric transducer 11 along signal path17. The piezoelectric transducer 11 generates a corresponding ultrasoundtest signal. Units 15 and 16 can also be provided in one single unit.

The test signal is provided as a short pulse, for example by a single 1MHz oscillation or by 10 such oscillations. In particular, the testsignal may be provided by a small number of oscillations with constantamplitude, thereby approximating a rectangular signal. The oscillationor the oscillations may have a sinusoidal shape, a triangular shape, arectangular shape or also other shapes.

The ultrasound test signal travels through the liquid in the pipe 12 tothe piezoelectric sensor 13. In FIG. 1, a direct signal path of theultrasound signal is indicated by an arrow 18. Likewise, a direct signalpath of the ultrasound signal in the reverse direction is indicated byan arrow 19. A response signal is picked up by the piezoelectric sensor13, sent to the second computation unit 16 along signal path 20, anddigitized by the second computation unit 16.

In a further step, a digital measuring signal is derived from thedigitized response signal. The derivation of the measurement refers to areversal of the digitized response signal with respect to time.According to further embodiments, the derivation comprises further stepssuch as a conversion to a reduced resolution in the amplitude range, abandwidth filtering of the signal to remove noise, such as low frequencynoise and high frequency noise. In particular, the step of bandwidthfiltering may be executed before the step of reversing the signal withrespect to time.

The signal reversal may be carried out in various ways, for example byreading out a memory area in reverse direction or by reversing the signof sinus components in a Fourier representation.

In one embodiment, a suitable portion of the digitized response signalis selected that contains the response from the direct signal. Theportion of the response signal is then turned around or is inverted withrespect to time. In other words, signal portions of the response signalthat are received later are sent out earlier in the inverted measuringsignal. If a signal is represented by a time ordered sequence ofamplitude samples, by way of example, the abovementioned signalinversion amounts to inverting or reversing the order of the amplitudesamples.

The resulting signal, in which the direction, or the sign, of time hasbeen inverted, is also referred to as an “inverted signal”. Theexpression “inverted” in this context refers to an inversion withrespect to the direction of time, and not to an inversion with respectto a value, such as the amplitude value.

FIGS. 10 to 19 show, by way of example digital signals according to thepresent specification.

In a flow meter according to one embodiment of the presentspecification, the same measuring signal is used for both directions 18,19, the downstream and the upstream direction, providing a simple andefficient arrangement. According to other embodiments, differentmeasuring signals are used for both directions. In particular, themeasuring signal may be applied to the original receiver of the testsignal. Such arrangements may provide benefits for asymmetric conditionsand pipe shapes.

A method of measuring a flow speed of a liquid through a pipe, whichuses the abovementioned-inverted signal as a measuring signal, comprisesthe following steps.

The abovementioned measuring signal is sent from the first computationunit 15 to the piezoelectric transducer 11 along signal path 17. Thepiezoelectric transducer 11 generates a correspondingultrasound-measuring signal. Examples for such a measuring signal areprovided in FIGS. 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34,37, and 38.

The ultrasound-measuring signal travels through the liquid in the pipe12 to the piezoelectric sensor 13. A response signal is picked up by thepiezoelectric sensor 13, sent to the second computation unit 16 alongsignal path 20, and digitized by the second computation unit 16.

The second computation unit 16 sends the digitized response signal tothe first computation unit 15. The first computation unit 15 determinesa time of flight of the received signal, for example by using one of themethods described further below.

A similar process is carried out for a signal travelling in the reversedirection 19, namely the abovementioned measuring signal is applied tothe downstream piezoelectric element 13, and a response signal ismeasured by the upstream piezoelectric element 11 to obtain an upstreamtime of flight TOF_up in the reverse direction 19. The first computationunit 15 determines a velocity of flow, for example according to theformula

${v = {\frac{c^{2}}{{2 \cdot L \cdot \cos}\mspace{14mu} \beta} \cdot \left( {{TOF}_{up} - {TOF}_{down}} \right)}},$

wherein L is the length of the direct path between the piezoelectricelements 11, 13, β is the angle of inclination of the direct pathbetween the piezoelectric elements 11, 13 and the direction of theaverage flow, and c is the velocity of sound in the liquid under thegiven pressure and temperature conditions.

The squared velocity of sound ĉ2 can be approximated to second order bythe expression

$c^{2} \approx \frac{L^{2}}{{TOF}_{up}*{TOF}_{down}}$

which leads to the formula

$v = {\frac{L}{2*\cos \mspace{14mu} \beta} \cdot \frac{{TOF}_{up} - {TOF}_{down}}{{TOF}_{up}*{TOF}_{down}}}$

Thereby, it is not necessary to determine temperature or pressure, whichin turn determine the fluid density and the sound velocity, or tomeasure the sound velocity or the fluid density directly. By contrast,the first order of the error does not cancel out for only onemeasurement direction.

Instead of using a factor 2·L·cos β, a proportionality constant can bederived from a calibration measurement with a known flow speed. Theproportionality constant of the calibration takes into account furthereffects such as flow profiles and contributions from sound waves thatwere scattered and did not travel along a straight line.

According to a further embodiment, the process of generating an impulsesignal, recording a response signal and deriving an inverted measuringsignal from the response signal is simulated in a computer. Relevantparameters, such as the pipe diameter of the pipe 12 and the sensorplacements are provided as input parameters to the simulation.

According to yet another embodiment, the measuring signal, which is tobe supplied to a transmitting piezoelectric element, is synthesizedusing a shape of a typical response signal to an impulse signal, such asthe signal shapes shown in FIGS. 37 and 38. For example, the measuringsignal may be provided by a 1 MHz sinusoidal oscillation, which isamplitude modulated with an envelope according to a Gaussian probabilityfunction having a half width of 10 microseconds. The half-width may bechosen as an input parameter, which depends on the actual arrangement,such as the pipe diameter and the sensor placement.

A flow meter according to the present specification may also be providedas a pre-defined flow meter in which the measuring signal is generatedduring a test run at a factory site, in particular when the flow meteris supplied together with a pipe section.

According to a simple embodiment of the present specification, a time offlight in upstream and in downstream direction is determined byevaluating a time of a peak amplitude of a received signal with respectto a sending time of the measuring signal. To achieve a higherprecision, the maximum may be determined using an envelope of thereceived signal. According to a further embodiment, the measurement isrepeated multiple times and an average time of flight is used.

According to a further embodiment of the present specification, the timeof flight of a signal is evaluated using a cross-correlation technique.In particular, the respective time shifts can be evaluated by crosscorrelating the received downstream or upstream signal with the receivedsignal at zero flow speed according to the formula:

${{{CCorr}(\tau)} = {\sum\limits_{t = {- \infty}}^{\infty}\; {{{Sig}_{Flow}(t)} \cdot {{Sig}_{NoFlow}\left( {t + \tau} \right)}}}},$

wherein Sig_Flow represents an upstream or downstream signal undermeasurement conditions, when there is a fluid flow through the pipe, andwherein Sig_NoFlow represents a signal under calibration conditions atzero flow. The infinite sum limits represent a sufficiently large timewindow [−T1, +T2]. In more general words, −T1 and +T2 do not need to besame and for practical reasons this can be advantageous for the flowmeter.

The time shift TOF_up-TOF_down is then obtained by comparing the time ofthe maximum of the upstream correlation function with the time of themaximum of the downstream correlation function. The envelope of thecorrelation function may be used to determine the location of themaximum more accurately.

In a further embodiment, a separate evaluation unit is provided betweenthe first computation unit 15 and the second computation unit 16, whichperforms the calculation of the signal arrival times and the flow speed.

In general, the measured signal of the acoustic sensor results from asuperposition of scattered signals and a direct signal. The scatteredsignals are reflected from the inner and outer walls of the pipe once ormultiple times including additional scattering processes within the pipewall. This is shown, by way of example, in FIG. 2.

The transducer configuration of FIG. 1 is a direct-line or “Z”configuration. Other arrangements, which make use of reflections on anopposite side of the pipe, are possible as well, such as the “V” and the“W” configuration. V and W configuration work based on reflections onthe pipe wall, which induce more scatterings than the Z configuration.The subject matter of the application will benefit from theseconfigurations as long as the paths are understood properly.

In a V-configuration, the two transducers are mounted on the same sideof the pipe. For recording a 45-degree reflection, they are placed abouta pipe diameter apart in the direction of the flow. The W-configurationmakes use of three reflections. Similar to the V-configuration, the twotransducers are mounted on the same side of the pipe. For recording asignal after two 45-degree reflections, they are placed two pipediameters apart in the direction of the flow.

FIG. 2 shows, by way of example a first acoustic signal “1”, whichtravels directly from the piezoelectric element 11 to the piezoelectricelement 13, a second acoustic signal “2”, which is scattered once at theperiphery of the pipe 12 and a third signal 3, which is scattered threetimes at the periphery of the pipe 12.

For simplicity, the scattering events are shown as reflections in FIGS.2 to 5 but the actual scattering process can be more complicated. Inparticular, the most relevant scattering occurs typically in the pipewall or at material that is mounted in front of the piezoelectrictransducers. FIG. 3 shows a view of FIG. 2 in flow direction in theviewing direction A-A.

FIGS. 4 and 5 show a second sensor arrangement in which a furtherpiezoelectric element 22 is positioned at a 45-degree angle to thepiezoelectric element 11 and a further piezoelectric element 23 ispositioned at a 45-degree angle to the piezoelectric element 13.

Furthermore, FIGS. 4 and 5 show direct or straight line, acoustic signalpaths for a situation in which the piezoelectric elements 11, 22 areoperated as piezo transducers and the piezoelectric elements 13, 23 areoperated as acoustic sensors. Piezoelectric element 23, which is on theback of the pipe 12 in the view of FIG. 4 is shown by a dashed line inFIG. 4.

FIGS. 6 to 9 show, in a simplified way, a method of generating ameasuring signal from a response of a test signal. In FIGS. 6 to 9,losses due to scattering are indicated by hatched portions of a signaland by arrows.

For the considerations of FIGS. 6 to 9, it is assumed that the acousticsignal only propagates along a straight line path, along a firstscattering channel with a time delay of Δt, and along a secondscattering channel with a time delay of 2Δt. Signal attenuation alongthe paths is not considered.

A test signal in the form of a rectangular spike is applied to thepiezoelectric element 11. Due to scattering, a first portion of thesignal amplitude is lost due to the first scattering path and appearsafter a time Δt, and a second portion of the signal amplitude is lostdue to the second scattering path and appears after a time 2Δt. Thisyields a signal according to the white columns in FIG. 7, which isrecorded at the piezoelectric element 13.

A signal processor inverts this recorded signal with respect to time andis applies the inverted signal to the piezoelectric element 11. The samescattering process as explained before now applies to all three-signalcomponents. As a result, a signal according to FIG. 9 is recorded at thepiezoelectric element 13, which is approximately symmetric.

In reality, the received signals will be distributed over time and thereoften is a “ballistic wave”, which has traveled through material of thepipe and arrives before the direct signal. This surface wave isdiscarded by choosing a suitable time window for generating the invertedmeasuring signal. Likewise, signals that stem from multiple reflectionsand arrive late can be discarded by limiting the time window and/or bychoosing specific parts of the signal.

The following table 1 shows measured time delays for a direct alignment,or, in other words, for a straight-line connection between clamped-onpiezoelectric elements on a DN 250 pipe in a plane perpendicular to thelongitudinal extension of the DN 250 pipe. The flow rate refers to aflow of water through the DN 250 pipe.

Herein “TOF 1 cycle” refers to an impulse such as the one shown in FIG.36, that is generated by a piezoelectric element, which is excited by anelectric signal with 1 oscillation having a 1 μs period. “TOF 10 cycle”refers to a signal that is generated by a piezoelectric element, whichis excited by an electric signal with 10 sinusoidal oscillations ofconstant amplitude having a 1 μs period.

Flowrate Method 21 m³/h 44 m³/h 61 m³/h TOF 1 cycle 7 ns 18 ns 27 ns TOF10 cycle 9 ns 19 ns 26 ns Time reversal 8 ns 18 ns 27 ns

The following table shows measured time delays for a 12 degreemisalignment against a straight line connection between clamped-onpiezoelectric elements in a DN 250 pipe in a plane perpendicular to thelongitudinal extension of the DN 250 pipe (see also FIGS. 48 & 49).

Flowrate Method 21 m³/h 44 m³/h 61 m³/h TOF 1 cycle 10 ns  21 ns 28 nsTOF 10 cycle 9 ns 17 ns 26 ns Time reversal 4 ns 12 ns 26 ns

FIGS. 9-27 show high resolution inverted signals and their respectiveresponse signals. The voltage is plotted in arbitrary units over thetime in microseconds.

The time axes in the upper Figures show a transmitting time of theinverted signal. The transmitting time is limited to the time windowthat is used to record the inverted signal. In the example of FIGS. 9-27the time window starts shortly before the onset of the maximum, whichcomes from the direct signal and ends 100 microseconds thereafter.

The time axes in the lower Figures are centered around the maximum ofthe response signals and extend 100 microseconds, which is the size ofthe time window for the inverted signal, before and after the maximum ofthe response signals.

FIGS. 28-35 show digitized inverted signals in a high resolution and in12, 3, 2 and 1 bit resolution in the amplitude range and theirrespective response signals. The voltage is plotted in Volt over thetime in microseconds. The signals of FIG. 28—25 were obtained for awater filled DN 250 pipe.

The length of the time window for the inverted signal is 450microseconds. Hence, the time window of FIGS. 28-35 is more than fourtimes larger than in the preceding FIGS. 9-27.

In FIGS. 28-35 it can be seen that even a digitization with 1 bitresolution produces a sharp spike. It can be seen that the spike becomeseven more pronounced for the lower resolutions. A possible explanationfor this effect is that in the example of FIGS. 28-35 the total energyof the input signal is increased by using a coarser digitization in theamplitude range while the response signal remains concentrated in time.

FIG. 36 shows a signal that is generated by a piezoelectric elementafter receiving an electric pulse that lasts for about 0.56microseconds, which is equivalent to a frequency of 3.57 MHz. Due to theinertia of the piezoelectric element, the maximum amplitude for thenegative voltage is smaller than for the positive voltage and there aremultiple reverberations before the piezoelectric element comes to rest.

FIG. 37 shows an electric signal that is applied to a piezoelectricelement, such as the upstream piezoelectric element 11 of FIG. 1. Thesignal of FIG. 37 is derived by forming an average of ten digitizedresponse signals to a signal of the type shown in FIG. 36 and timereversing the signal, wherein the response signals are received by apiezoelectric element such as the downstream piezoelectric element 13 ofFIG. 1.

In the example of FIG. 37, the digitized signals are obtained by cuttingout a signal portion from the response signal that begins approximately10 microseconds before the onset of envelope of the response signal andthat ends approximately 55 microseconds behind the envelope of theresponse signal. The envelope shape of the response signal of FIG. 37 issimilar to the shape of a Gaussian probability distribution, or, inother words, to a suitable shifted and scaled version of exp(−x̂2).

FIG. 38 shows a portion of a response signal to the signal shown in FIG.37, wherein the signal of FIG. 37 is applied to a first piezoelectricelement, such as the upstream piezoelectric element 11, and is receivedat a second piezoelectric element, such as the downstream piezoelectricelement 13 of FIG. 1.

FIG. 39 shows a an upstream cross correlation function and a downstreamcross correlation function, which are obtained by cross correlating theupstream signal and the downstream signal of the arrangement of FIG. 1with a signal obtained at zero flow, respectively.

FIG. 40 shows a sectional enlargement of FIG. 39. Two position markersindicate the positions of the respective maxima of the upstream anddownstream cross correlation function. The time difference between themaxima is a measure for the time difference between the upstream and thedownstream signal.

FIG. 41 shows a response signal, which was obtained under similarconditions as for the response signal of FIG. 37. Different from thearrangement of FIG. 37, the piezoelectric elements are misaligned by 12degrees against a straight-line arrangement along the perimeter of thepipe. This offset is shown in the inset of FIG. 41. FIG. 41 shows thateven under misalignment conditions there is a reasonably well definedresponse signal.

FIGS. 42 to 45 show, by way of example, different arrangements ofclamp-on piezoelectric transducers for which a flow measurementaccording to the present specification can be used. Especially forclamp-on transducers a flow measurement method according to the presentspecification may lead to an improvement of the signal to noise ratio inthe arrangements of FIGS. 42 to 45 or in other, similar transducerarrangements. Furthermore, the flow measurement method may provideenergy savings by providing an increased signal amplitude of theresponse signal for a given sending signal power. Thereby, a signalsending power can be reduced.

FIGS. 42 to 45 are aligned such that a gravity force on a liquid in thepipe 12 points downwards. However, arrangements, which are rotatedrelative to the arrangements of FIGS. 42 to 45, may also be used. Theviewing direction of FIGS. 42 to 45 is along the longitudinal axis ofthe pipe 12. An upstream or downstream position of a transducer is notindicated in FIGS. 42 to 45.

In the arrangement of FIG. 42, an array of five piezoelectric elements31-35 is provided in a first location and a further piezoelectricelement 36 is placed upstream or downstream of the first location. Thearray of piezoelectric elements 31-35 may be used to obtain apre-determined wave front and to achieve an improved focusing of anacoustic wave in a pre-determined direction, when the array of fiveelements 31-35 is used as a transmitter and the further element 36 isused as a receiver.

In the arrangement of FIG. 43, a single piezoelectric element 37 isprovided in a first location and an array of five piezoelectric elements38-42 is placed upstream or downstream of the first location. The arrayof piezoelectric elements 38-42 may be used to obtain an improvedrecording of the wave front of the response signal. The improvedrecording can then be used to obtain an improved flow-measuring signal,which is then applied to the single piezoelectric element 37.

FIG. 44 shows an arrangement of two piezoelectric elements 43, 44wherein one element is placed downstream with respect to the other. Adistance d of the connection line between the piezoelectric elements 43,44 to the symmetry axis of the pipe 12 is about half the radius of thepipe 12, such that a flow layer at a distance d to the central axis ofthe pipe 12 can be measured.

Especially for clamp-on transducers, such as the piezoelectric elements43, 44 shown in FIG. 44, the flow measurement according to the presentspecification provides an improved signal at the receiving piezoelectricelement 44, 43 through beam forming.

FIG. 45 shows an arrangement of eight piezoelectric elements 45-52,which are spaced at 45 degrees apart. Several arrangements are possiblewith respect to upstream-downstream placements.

In one arrangement, the sensors locations alternate between upstream anddownstream along the perimeter, for example 45, 47, 49, 51 upstream and46, 48, 50, 52 downstream.

In another arrangement, first four consecutive elements, such as 45-48,along the perimeter are placed upstream or downstream relative to theother four elements, such as 49-52. In a further arrangement with 16piezoelectric elements, all the piezoelectric elements 45-52 of FIG. 45are placed in one plane and the arrangement of FIG. 45 is repeated inupstream or downstream direction.

FIG. 46 shows, by way of example, a flow measurement device 60 formeasuring a flow in the arrangement in FIG. 1 or other arrangementsaccording to the specification. In the arrangement of FIG. 1, the flowmeasurement device 60 is provided by the first and second computationunits 15, and 16.

The flow measurement device 60 comprises a first connector 61 forconnecting a first piezoelectric transducer and a second connector 62for connecting a second piezoelectric transducer. The first connector 61is connected to a digital to analog converter (DAC) 64 over amultiplexer 63. The second connector 62 is connected to an analog todigital converter 65 over a demultiplexer 66.

The ADC 65 is connected to a signal selection unit 67, which isconnected to a signal inversion unit 68, which is connected to a bandpass filter 69, which is connected to a computer readable memory 70.Furthermore, the ADC 65 is connected to a velocity computation unit 71.

The DAC 64 is connected to an impulse signal generator 72 and ameasuring signal generator 73. The measuring signal generator isconnected to the impulse generator 72 over a command line 74. Thevelocity computation unit 71 is connected to the measuring signalgenerator 73 via a second command line 75.

In general, the impulse signal generator 72 and the measuring signalgenerator comprise hardware elements, such as an oscillator, andsoftware elements, such as an impulse generator module and a measuringsignal generator module. In this case, the command lines 74, 75 may beprovided by software interfaces between respective modules.

During a signal-generating phase, the impulse signal generator sends asignal to the DAC 64, the selection unit 67 receives a correspondingincoming signal over the ADC 65 and selects a portion of an incomingsignal. The inversion unit 68 inverts the selected signal portion withrespect to time, the optional bandpass filter 69 filters out lower andupper frequencies and the resulting measuring signal is stored in thecomputer memory 70. When the word “signal” is used with reference to asignal manipulation step, it may in particular refer to a representationof a signal in a computer memory.

In particular, a signal representation can be defined by value pairs ofdigitized amplitudes and associated discrete times. Otherrepresentations comprise, among others, Fourier coefficients, waveletcoefficients and an envelope for amplitude modulating a signal.

FIG. 47 shows a second embodiment of a flow measurement device 60′ formeasuring a flow in the arrangement in FIG. 1 or other arrangementsaccording to the specification. The flow measurement device 60′comprises a direct digital synthesizer

(DDS) 76. For simplicity, only the components of the DDS 76 are shown.The DDS 76 is also referred to as an arbitrary waveform generator (AWG).

The DDS 76 comprises a reference oscillator 77, which is connected to afrequency controller register 78, a numerically controlled oscillator(NCO) 79 and to the DAC 64. An input of the NCO 79 for N channels isconnected to an output of the frequency control register 78. An input ofthe DAC 64 for M channels is connected to the NCO 79 and an input of areconstruction low pass filter is connected to the DAC 64. By way ofexample, a direct numerically controlled oscillator 79 with a clockfrequency of 100 MHz may be used to generate an amplitude modulated 1MHz signal.

An output of the reconstruction low pass filter 80 is connected to thepiezoelectric transducers 11, 13 of FIG. 1.

Due to the inertia of an oscillator crystal, it is often advantageous touse an oscillator with a higher frequency than that of a carrier wave inorder to obtain a predetermined amplitude modulated signal, for exampleby using a direct digital synthesizer, as shown in FIG. 47.

In particular, the method steps of storing a digital representation of asignal and performing operations such as selection a signal portion,time reversing a signal and filtering a signal may be interchanged. Forexample, a signal may be stored in a time inverted form or it may beread out in reverse order to obtain a time inverted signal.

While the present invention is explained with respect to a round DN 250pipe, it can be readily applied to other pipe sizes or even to otherpipe shapes. Although the embodiments are explained with respect toclamp-on transducers, wet transducers, which protrude into a pipe, maybe used as well.

FIGS. 48 and 49 show an asymmetric transducer arrangement, wherein asecond transducer is offset by 12 degrees with respect to a symmetryaxis of the conduit 12.

FIG. 50 shows a one cycle measuring signal of a time of flightmeasurement, and FIG. 51 shows a ten cycle measuring signal of a time offlight measurement. The signals shown in FIGS. 50 and 51 can be used fora time of flight measurement. Furthermore, the signals can also be usedto generate a measurement signal according to the present specificationusing an inversion with respect to time of a received response signal,such as the response signals of FIGS. 52 and 53.

FIG. 52 shows an example of a measuring signal that is derived from atime-reversed signal, which is stored at a low resolution.

FIGS. 53 to 55 show response signals to the respective signals of FIGS.50 to 52. The response signal is picked up by a receiving transducer 11,13 of the asymmetric arrangement of FIGS. 48, 49 in response to a signalof a sending transducer, which is excited by the signal of FIG. 50.

In particular, FIG. 53 shows a response signal of the signal of FIG. 50,FIG. 54 shows a response signal of the signal of FIG. 51 for thearrangement of FIGS. 48 and 49 and FIG. 55 shows a response signal ofthe Signal of FIG. 52 for the arrangement of FIGS. 48 and 49. In theexamples shown, the response signal is more concentrated in time, has ahigher amplitude and has a more well-defined envelope as compared to thesignals of FIGS. 52 and 53.

The result of FIG. 55 demonstrates that the benefits of inversion of theimpulse response with respect to time, which allow, among others, to usesmaller energy signals, can be retained for coarse-grained resolutionand asymmetric transducer arrangements.

The results of FIG. 55 demonstrate that using an inverted signal withrespect to time according to the present specification is able toprovide short time delays as compared with conventional time of flightDoppler shift measurements using a signal with 1 or 10 oscillationcycles. FIG. 55 as a result of the arrangement shown in FIG. 48 and FIG.49 furthermore shows that a measuring signal according to the presentspecification can be used for beam shaping purposes.

Table 2 shows results time delays for the asymmetric arrangement shownin FIGS. 48, 49 and for respective flow rates of 21, 44, and 61 cubicmetres per hour.

Flowrate Methods 21 m3/h 44 m3/h 61 m3/h TOF 1 cycle 10 ns  21 ns 28 nsTOF 10 cycle 9 ns 17 ns 26 ns inverted signal 4 ns 12 ns 26 ns

The FIGS. 56 to 59 illustrate further examples of beam shapingapplications. In general, there are Σ_(i=1) ^(N-1)i=N*(N−1)/2 directtransmission channels between N transducers not considering reflectionsat the pipe walls, which are provided on a conduit. These transmissionchannels have in general different properties and lead to differentresponse signals.

In the event that all of the N transducers are mounted at differentheights with respect to a flow direction or a longitudinal direction ofthe conduit, all of these transmission channels can be used for flowmeasurements. A signal propagation between transducers that isperpendicular to the mean flow is in general not useful for capturingflow velocity components but can still be used to determinecontaminations and material changes of the conduit and changes in theproperties of the transducers and their coupling to the conduit.

A TOF flow measurement comprises a measurement in both directions withrespect to a given transmission channel between two of the transducers.A TOF flow measurement that involves transmission channels between afirst transducer and N−1 other transducers requires at least Nconsecutive measurements: a first measurement with a measurement signalapplied to the first transducer and N−1 consecutive measurements withmeasurement signals applied to each one of the N−1 other transducers.

In general, the required measurement signals are different for eachtransmission channel and separate forward and a backward measurementsare needed for each transmission channel. Thus, 2×(N−1) measurements arerequired. For example, max 2×(3−1)=4 measurements are possible, but notnecessarily required, in the example of FIG. 57.

The signals of FIGS. 41 and 55 are produced by transducers, whichradiate mainly in a preferred direction, with a maximum angle of about12 degrees to both sides of the preferred direction. The directionalityof the transducers is achieved, among others, by adjusting the form ofthe transducers and their attachment to the conduit. Depending on theangle between the transducers, not all paths may yield a sufficientlystrong signal at a receiver side, especially if the sender has a highdirectionality. Applying the common known techniques only results asshown in FIG. 53 and FIG. 54 can be achieved with are typically toonoisy for establishing flow measurements. However with the proposedmethod using inverted measurements signals, sufficiently good signalslike those shown in FIG. 55 can be achieved.

The use of a measuring signal according to the application, which uses areversion with respect to time makes it possible to provide transducerswith less directionality. The measuring signal focuses the signal energyat the receiver and the received signal is still strong enough.

Similar to a measurement using just two transducers or just onetransmission channel, the flow measurement can be performed using apre-determined measuring signal or a signal that is obtained by a priorcalibration. During the calibration step, the measuring signals arederived from response signals to the impulse signals. According to oneexample, an impulse signal is applied to a transducer to obtain one ormore response signals at the other transducers. The measurement signalsare derived by applying an inversion with respect to time to theresponse signals or a portion thereof.

In one example, in which there are four measurement paths, consecutivemeasurements are taken along the first path, the second path, the thirdpath and the fourth path. The consecutive measurements are used toderive an overall flow and/or flows in at a predetermined layer orposition.

One or more flow speeds can then be derived by comparing themeasurements with a pre-determined flow profile. By way of example, thepre-determined flow profile can be obtained by a simulation. In anotherembodiment, a flow speed for a specific layer or position is estimatedby using results from one or more measuring signals and known methods tocalculate the flow profile. In one embodiment, an overall volume flow isderived by applying a calculated or simulated flow profile to a crosssection area of the conduit.

FIGS. 56 and 57 illustrate a time of flight flow measurement using threetransducers and two transmission paths.

FIGS. 58 and 59 show a pressure measurement in a two-transducerarrangement. The pressure scale of FIG. 59 is displayed in arbitraryunits (a.u.).

A measurement signal according to the present application is applied tothe first transducer 11 and the resulting pressure distribution ismeasured at the periphery of the conduit 12. The transducers 11, 13 areoffset in the longitudinal direction, similar to the arrangement of FIG.1.

In a first example, a measuring signal which adapted to the signal pathbetween the transducer 11 and the transducer 13 is sent from thetransducer 11 to the opposite transducer 13 and the resulting pressuredistribution is measured. This yields a curve similar to the pressuredistribution 90 of FIG. 59, which has a peak at the position of thetransducer 13.

In a second example, a measuring signal which adapted to the signal pathbetween the transducer 11 and the transducer 13 is sent from thetransducer 11 to transducer 13′ and the resulting pressure distributionis measured. Different from the first arrangement, the transducer 13′ isoffset by an angle of 45 degrees with respect to a connection linethrough the first transducer and the center of the conduit 12. Even inthis situation, the resulting pressure distribution is peaked around theposition of the transducer 13′ and consequently the energy of the signalis concentrated around the position of the transducer 13′

Thus, a measuring signal according to the application, which isobtaining using a reversal with respect to time of a signal between therespective transducers, leads to a pressure signal that is not onlyconcentrated in time, as shown in the respective second Figure of theFigure sets 10-35, but the resulting pressure distribution is alsoconcentrated in space.

By using a standard signal, such as an impulse signal, a concentrationin space can still be achieved, but only at a fixed location close tothe opposite side of the sending transducer. However, by using a signalaccording to the present specification, which comprises a time reversedportion the peak of the pressure concentration can be moved.

The ultrasonic transducers 11, 13, 23 of FIGS. 56-59 can be provided bymounted transducers, which are mounted to the outside of the conduit, orwet by transducers, which are protruding into the interior of theconduit 12 from outside of the conduit 12.

FIG. 60 show an example of determining whether a test device uses thesame method of flow measurement as a verification device. In a firststep, the verification device selects a test impulse signal. Forexample, this may comprise the selection of a signal shape forperforming an amplitude modulation of a sine wave out of a set of storedsignal shapes, such as rectangular shape, a sinusoidal shape, a sawtooth shape etc.

In a further step, the test impulse signal is applied to a firsttransducer. In a further step, a corresponding test response signal isreceived at the second transducer. In further steps, the test responsesignal, or a portion of it, is inverted and a test measuring signal isderived. The transducers to which the verification device is connectedare preferably the transducers of the test device.

In a further step, the test measuring signal is compared with the actualmeasuring signal of the test device. If the test measuring signal issimilar to the measuring signal of the test device, it is decided thatthe test device uses the same method as the verification device.Alternatively or in addition, the verification device can apply the testmeasuring signal to a transducer, receive a corresponding test responsesignal and compare this test response signal with the response signal tothe measuring signal of the test device.

The verification device may receive or measure the signals of the testdevice as electric signals via tapping a wire connection of the testdevice to the transducers or, alternatively, the signals can be measuredby placing a microphone in the conduit and receiving a signal of themicrophone.

If the signals are not similar, the same process is repeated withfurther available test impulse signals to see whether one of the testimpulse signals leads to a test measuring signal and/or a responsesignal to it which resembles the measuring signal and/or the responsesignal to it. In the event that an impulse signal of the test device isavailable, the verification device may choose the available impulsesignal or a similar impulse signal instead of testing various testimpulse signals or it may narrow down the selection of test impulsesignals.

For a test device which uses several signal paths and/or combinations ofpairs of sending and receiving transducers for the flow measurement, theverification device repeats the method of FIG. 60 for the multiplesignal paths and/or for the multiple transducer combinations and therespective measuring signals and/or response signals to the measuringsignals are compared.

Although the above description contains much specificity, these shouldnot be construed as limiting the scope of the embodiments but merelyproviding illustration of the foreseeable embodiments. The method stepsmay be performed in different order than in the provided embodiments,and the subdivision of the measurement device into processing units andtheir respective interconnections may be different from the providedembodiments.

Especially, the above stated advantages of the embodiments should not beconstrued as limiting the scope of the embodiments but merely to explainpossible achievements if the described embodiments are put intopractice. Thus, the scope of the embodiments should be determined by theclaims and their equivalents, rather than by the examples given.

The embodiments of the present specification can also be described withthe following lists of elements being organized into items. Therespective combinations of features which are disclosed in the item listare regarded as independent subject matter, respectively, that can alsobe combined with other features of the application.

Embodiment 1

A method for determining a flow speed of a fluid in a fluid conduitcomprising:

-   -   providing the fluid conduit with a fluid that has a        predetermined velocity with respect to the fluid conduit,    -   providing the fluid conduit with a first ultrasonic transducer,        a second ultrasonic transducer and a third ultrasonic        transducer, wherein respective connection lines between the        first ultrasonic transducer, the second ultrasonic transducer        and the third ultrasonic transducer extend outside of a symmetry        axis of the fluid conduit,    -   applying a first measuring signal to the first ultrasonic        transducer, and    -   measuring a first response signal of the first measuring signal        at the second ultrasonic transducer,    -   applying a second measuring signal to the first ultrasonic        transducer,    -   measuring a second response signal of the second measuring        signal at the third ultrasonic transducer,

wherein the first measuring signal and the second measuring signalrespectively comprise a reversed signal portion with respect to time ofa response signal of a corresponding impulse signal or of a signalderived therefrom,

-   -   deriving a flow speed of the fluid from at least one of the        first response signal and the second response signal.

Embodiment 2

The method according to embodiment 1, comprising:

-   -   applying a first reverse direction measuring signal to the        second ultrasonic transducer, and    -   measuring a first reverse direction response signal of the first        reverse direction measuring signal at the second ultrasonic        transducer,    -   applying a second reverse direction measuring signal to the        third ultrasonic transducer,    -   measuring a second reverse direction response signal of the        second reverse direction measuring signal at the first        ultrasonic transducer,

wherein the first reverse direction measuring signal and the secondreverse direction measuring signal respectively comprise a reversedsignal portion with respect to time of a response signal of acorresponding impulse signal or of a signal derived therefrom,

-   -   deriving a flow speed of the fluid from at least one of the        first response signal, the first reverse direction response        signal, the second response signal and the second reverse        direction response signal.

Embodiment 3

The method according to embodiment 1 or embodiment 2, comprising:

-   -   applying a third measuring signal to the second ultrasonic        transducer,    -   measuring a third response signal of the second measuring signal        at the third ultrasonic transducer,

wherein the third measuring signal comprises a reversed signal portionwith respect to time of a response signal of a corresponding impulsesignal or of a signal derived therefrom

-   -   deriving at least one flow speed of the fluid from the third        response signal.

Embodiment 4

The method according to embodiment 3, comprising:

-   -   applying a third reverse direction measuring signal to the third        ultrasonic transducer,    -   measuring a third reverse direction response signal of the third        reverse direction measuring signal at the second ultrasonic        transducer,

wherein the third reverse direction measuring signal comprises areversed signal portion with respect to time of a response signal of acorresponding impulse signal or of a signal derived therefrom

-   -   deriving at least one flow speed of the fluid from the third        response signal and the third reverse direction response signal.

Embodiment 5

A method for determining a flow speed of a fluid in a fluid conduitcomprising:

-   -   providing the fluid conduit with a fluid that has a        predetermined velocity with respect to the fluid conduit,    -   providing the fluid conduit with a first ultrasonic clamp-on        transducer and a second ultrasonic clamp-on transducer,

wherein a connection line between the first ultrasonic clamp-ontransducer and the second ultrasonic clamp-on transducer extends outsideof a symmetry axis of the fluid conduit,

-   -   applying a measuring signal to the first ultrasonic clamp-on        transducer,    -   measuring a response signal of the measuring signal at the        second ultrasonic clamp-on transducer,

wherein the measuring signal comprises a reversed signal portion withrespect to time of a response signal of a corresponding impulse signalor of a signal derived therefrom,

-   -   deriving a flow speed of the fluid from the response signal.

Embodiment 6

The method according to embodiment 5, comprising:

-   -   applying a reverse direction measuring signal to the second        ultrasonic clamp-on transducer,    -   measuring a reverse direction response signal of the measuring        signal at the first ultrasonic clamp-on transducer,

wherein the measuring signal comprises a reversed signal portion withrespect to time of a response signal of a corresponding impulse signalor of a signal derived therefrom,

-   -   deriving a flow speed of the fluid from the response signal.

Embodiment 7

The method according to one of the preceding embodiments, wherein thesignal portion that is used to derive the respective measuring signalscomprises a first portion around a maximum amplitude of a responsesignal and a trailing signal portion, the trailing signal portionextending in time behind the arrival time of the maximum amplitude.

Embodiment 8

The method according to one of the preceding embodiments, comprisingprocessing of at least one of the response signals for determining achange in the wall thickness of the conduit or for determining materialcharacteristics of the conduit walls by determining longitudinal andtransversal sound wave characteristics.

Embodiment 9

The method according to embodiment 1, comprising:

-   -   providing the fluid conduit with a fluid,    -   providing a first impulse signal to one of the first or the        second ultrasonic transducer,    -   receiving a first response signal of the first impulse signal at        the other one of the first or the second ultrasonic transducer,    -   providing a second impulse signal to one of the first or the        third ultrasonic transducer,    -   receiving a second response signal of the second impulse signal        at the other one of the first or the third ultrasonic        transducer,    -   deriving the first measuring signal from the first response        signal,    -   deriving the second measuring signal from the second response        signal,

the derivation of the respective first and second measuring signalscomprising selecting a signal portion of the respective first and secondresponse signals or of a signal derived therefrom and reversing thesignal portion with respect to time,

-   -   storing the first measuring signal and the second measuring        signal for later use.

Embodiment 10

The method according to embodiment 5, comprising:

-   -   providing the fluid conduit with a fluid,    -   providing an impulse signal to one of the first ultrasonic        clamp-on transducer and the second ultrasonic clamp-on        transducer,    -   receiving a response signal of the impulse signal at the other        one of the first ultrasonic clamp-on transducer and the second        ultrasonic clamp-on transducer,    -   deriving the measuring signal from the response signal,

the derivation of the measuring signal comprising selecting a signalportion of the respective response signal or of a signal derivedtherefrom and reversing the signal portion with respect to time,

-   -   storing the measuring signal for later use.

Embodiment 11

The method according to embodiment 9 or embodiment 10, comprising

-   -   repeating the steps of applying an impulse signal and receiving        a corresponding response signal multiple times, thereby        obtaining a plurality of response signals,    -   deriving the respective measuring signal from an average of the        received response signals.

Embodiment 12

The method according to one of the embodiments 9 to 11,

wherein the derivation of the respective measuring signal comprisesdigitizing the corresponding response signal or a signal derivedtherefrom with respect to amplitude.

Embodiment 13

The method according to embodiment 12, comprising increasing thebit-resolution of the digitized signal for increasing an amplitude of aresponse signal to the respective measuring signal.

Embodiment 14

The method according to embodiment 12, comprising decreasing thebit-resolution of the digitized signal for increasing an amplitude of aresponse signal to the respective measuring signal.

Embodiment 15

The method according to embodiment 12, wherein the bit resolution of thedigitized signal with respect to the amplitude is a low bit resolution.

Embodiment 16

A computer readable program code comprising computer readableinstructions for executing the method according to one of embodiments 1to 15.

Embodiment 17

A computer readable memory, the computer readable memory comprising thecomputer readable program code of embodiment 16.

Embodiment 18

An application specific electronic component, which is operable toexecute the method according to one of the embodiments 1 to 15.

Embodiment 19

A device for measuring a flow speed of a fluid in a conduit with atravel time ultrasonic flow meter, comprising

-   -   a first connector for connecting a first ultrasonic element,    -   a second connector for connecting a second ultrasonic element,    -   a third connector for connecting a third ultrasonic element,    -   a transmitting unit for sending impulse signals and for sending        measuring signals,    -   a receiving unit for receiving response signals,    -   a processing unit for deriving a first measuring signal from a        first inverted signal, for deriving a second measuring signal        from a second inverted signal and for storing the first        measuring signal and the second measuring signal,

wherein the derivation of the first inverted signal and of the secondinverted signal comprises reversing a signal portion of a responsesignal of a corresponding impulse signal or of a signal derivedtherefrom with respect to time,

and wherein the processing unit, the transmitting unit and the receivingunit are operative to apply the first measuring signal to the firstconnector, and

to receive a first response signal of the first measuring signal at thesecond connector,

to apply a second measuring signal to the first connector,

to receive a second response signal of the second measuring signal atthe third connector,

and to derive a flow speed of the fluid from at least one of the firstresponse signal and the second response signal.

Embodiment 20

A device for measuring a flow speed of a fluid in a conduit with atravel time ultrasonic flow meter, comprising

-   -   a first connector,    -   a first ultrasonic clamp-on transducer which is connected to the        first connector,    -   a second connector,    -   a second ultrasonic clamp-on transducer which is connected to        the second connector,

a portion of a conduit, the first ultrasonic clamp-on transducer beingmounted to the conduit portion at a first location,

and the second ultrasonic clamp-on transducer being mounted to theconduit portion at a location,

wherein respective connection lines between the first ultrasonicclamp-on transducer and the second clamp-on ultrasonic transducer extendoutside of a symmetry axis of the fluid conduit,

-   -   a transmitting unit for sending impulse signals and for sending        measuring signals,    -   a receiving unit for receiving response signals,    -   a processing unit for deriving a measuring signal from an        inverted signal, wherein the derivation of the inverted signal        comprises reversing a signal portion of a response signal of a        corresponding impulse signal or of a signal derived therefrom        with respect to time,

and wherein the processing unit, the transmitting unit and the receivingunit are operative

to apply the measuring signal to the first connector,

to receive a response signal of the first measuring signal at the secondconnector,

and to derive a flow speed of the fluid from the response signal.

Embodiment 21

The device of embodiment 20, further comprising:

-   -   a D/A converter, the D/A converter being connected to the first        connector,    -   an A/D converter, the A/D converter being connected to the        second connector,    -   a computer readable memory for storing the measuring signal.

Embodiment 22

The device of embodiment 20, further comprising a selection unit, theselection unit being operative to select a portion of a receivedresponse signal to the impulse signal or a signal derived therefrom, andan inverting unit, the inverting unit being operative to invert theselected portion of the received response signal with respect to time toobtain the inverted signal.

Embodiment 23

The device according to embodiment 20, the device comprising

a direct digital signal synthesizer, the direct digital signalsynthesizer comprising the ADC,

a frequency control register, a reference oscillator, a numericallycontrolled oscillator and a reconstruction low pass filter, the ADCbeing connectable to the first and the second connector over thereconstruction low pass filter.

Embodiment 24

The device according to embodiment 20, comprising a portion of aconduit, the first ultrasonic transducer being mounted to the conduitportion at a first location,

-   -   and the second ultrasonic transducer being mounted to the        conduit portion at a second location.

Embodiment 25

A method for determining whether a test device is measuring a flow speedof a fluid in a fluid conduit according to one of the embodiments 1 to5, comprising:

-   -   providing the fluid conduit with a fluid that has a        pre-determined velocity with respect to the fluid conduit,    -   providing the fluid conduit with a first ultrasonic transducer        and a second ultrasonic transducer,    -   applying a test impulse signal to the first ultrasonic        transducer of the test device,    -   receiving a test response signal of the test impulse signal at        the second ultrasonic transducer of the test device,    -   deriving a test measuring signal from the test response signal        the derivation of the test measuring signal comprising reversing        the respective first or second response signal, or a portion        thereof, with respect to time,    -   comparing the first test measuring signal with a first measuring        signal that is emitted at a transducer of the test device,    -   wherein it is determined that the test device is using a method        to determine a flow speed of a fluid in a fluid conduit        according to one of the items 1 to 5, if the first test        measuring signal and the first measuring signal are similar.

Embodiment 26

Method according to embodiment 25, comprising:

-   -   providing the fluid conduit with a third ultrasonic transducer,    -   applying a test impulse signal to the first ultrasonic        transducer of the test device or to the second ultrasonic        transducer of the test device,    -   receiving a second test response signal of the test impulse        signal at the at the third ultrasonic transducer of the test        device,    -   deriving a second test measuring signal from the second test        response signal,    -   comparing the second test measuring signal with a second        measuring signal that is emitted at a transducer of the test        device, wherein it is determined that the test device is using a        method to determine a flow speed of a fluid in a fluid conduit        according to item 1, if the first test measuring signal and the        first measuring signal are similar.

REFERENCE

-   10 flow meter arrangement-   11 upstream piezoelectric element-   12 pipe-   13 downstream piezoelec-tric element-   14 direction of average flow-   15 first computation unit-   16 second computation unit-   17 signal path-   20 signal path-   22 piezoelectric element-   23 piezoelectric element-   31-52 piezoelectric elements-   60, 60′ flow measurement device-   61 first connector-   62 second connector-   63 multiplexer-   64 DAC-   65 ADC-   66 demultiplexer-   67 signal selection unit-   68 signal inversion unit-   69 bandpass filter-   70 memory-   71 velocity computation unit-   72 impulse signal generator-   73 measuring signal generator-   74 command line-   75 command line-   76 DDS-   77 reference oscillator-   78 frequency controller register-   79 numerically controlled oscillator-   80 low pass filter

1. A method for determining a flow speed of a fluid in a fluid conduitcomprising: providing the fluid conduit with a fluid that has apredetermined velocity with respect to the fluid conduit providing thefluid conduit with a first ultrasonic transducer, a second ultrasonictransducer and a third ultrasonic transducer, wherein respectiveconnection lines between the first ultrasonic transducer, the secondultrasonic transducer and the third ultrasonic transducer extend outsideof a symmetry axis of the fluid conduit, applying a first measuringsignal to the first ultrasonic transducer, and measuring a firstresponse signal of the first measuring signal at the second ultrasonictransducer, applying a second measuring signal to the first ultrasonictransducer, measuring a second response signal of the second measuringsignal at the third ultrasonic transducer, wherein the first measuringsignal and the second measuring signal respectively comprise a reversedsignal portion with respect to time of a response signal of acorresponding arbitrary waveform generator signal or of a signal derivedtherefrom, deriving a flow speed of the fluid from at least one of thefirst response signal and the second response signal.
 2. The method ofclaim 1, further comprising: applying a first reverse directionmeasuring signal to the second ultrasonic transducer, and measuring afirst reverse direction response signal of the first reverse directionmeasuring signal at the first ultrasonic transducer, applying a secondreverse direction measuring signal to the third ultrasonic transducer,measuring a second reverse direction response signal of the secondreverse direction measuring signal at the first ultrasonic transducer,wherein the first reverse direction measuring signal and the secondreverse direction measuring signal respectively comprise a reversedsignal portion with respect to time of a response signal of acorresponding arbitrary waveform generator signal or of a signal derivedtherefrom, deriving a flow speed of the fluid from at least one of thefirst response signal, the first reverse direction response signal, thesecond response signal and the second reverse direction response signal.3. The method of claim 1, further comprising: applying a third measuringsignal to the second ultrasonic transducer, measuring a third responsesignal of the second measuring signal at the third ultrasonictransducer, wherein the third measuring signal comprises a reversedsignal portion with respect to time of a response signal of acorresponding arbitrary waveform generator signal or of a signal derivedtherefrom deriving at least one flow speed of the fluid from the thirdresponse signal.
 4. The method of claim 3, further comprising: applyinga third reverse direction response signal to the third ultrasonictransducer, measuring a third reverse direction response signal of thethird reverse direction measuring signal at the second ultrasonictransducer, wherein the third reverse direction measuring signalcomprises a reversed signal portion with respect to time of a responsesignal of a corresponding arbitrary waveform generator signal or of asignal derived therefrom deriving at least one flow speed of the fluidfrom the third response signal at the third reverse direction responsesignal.
 5. A method for determining a flow speed of a fluid in a fluidconduit comprising: providing the fluid conduit with a fluid that has apredetermined velocity with respect to the fluid conduit, providing thefluid conduit with a first ultrasonic clamp-on transducer and a secondultrasonic clamp-on transducer, wherein a connection line between thefirst ultrasonic clamp-on transducer and the second ultrasonic clamp-ontransducer extends outside of a symmetry axis of the fluid conduit,applying a measuring signal to the first ultrasonic clamp-on transducer,measuring a response signal of the measuring signal at the secondultrasonic clamp-on transducer, wherein the measuring signal comprises areversed signal portion with respect to time of a response signal of acorresponding arbitrary waveform generator signal or of a signal derivedtherefrom, deriving a flow speed of the fluid from the response signal.6. The method of claim 5, further comprising: applying a reversedirection measuring signal to the second ultrasonic clamp-on transducer,measuring a reverse direction response signal of the measuring signal atthe first ultrasonic clamp-on transducer, wherein the measuring signalcomprises a reversed signal portion with respect to time of a responsesignal of a corresponding arbitrary waveform generator signal or of asignal derived therefrom, deriving a flow speed of the fluid from theresponse signal.
 7. The method of claim 1, wherein the signal portionthat is used to derive the respective measuring signals comprises afirst portion around a maximum amplitude of a response signal and atrailing signal portion, the trailing signal portion extending in timebehind the arrival time of the maximum amplitude.
 8. The method of claim5, further comprising processing of at least one of the response signalsfor determining a change in the wall thickness of the conduit or fordetermining material characteristics of the conduit walls by determininglongitudinal and transversal sound wave characteristics.
 9. The methodof claim 1, further comprising: providing the fluid conduit with afluid, providing a first arbitrary waveform generator signal to one ofthe first or the second ultrasonic transducer, receiving a firstresponse signal of the first arbitrary waveform generator signal at theother one of the first or the second ultrasonic transducer, providing asecond arbitrary waveform generator signal to one of the first or thethird ultrasonic transducer, receiving a second response signal of thesecond arbitrary waveform generator signal at the other one of the firstor the third ultrasonic transducer, deriving the first measuring signalfrom the first response signal, deriving the second measuring signalfrom the second response signal, the derivation of the respective firstand second measuring signals comprising selecting a signal portion ofthe respective first and second response signals or of a signal derivedtherefrom and reversing the signal portion with respect to time, storingthe first measuring signal and the second measuring signal for lateruse.
 10. The method of claim 5, further comprising: providing the fluidconduit with a fluid, providing an arbitrary waveform generator signalto one of the first ultrasonic clamp-on transducer and the secondultrasonic clamp-on transducer, receiving a response signal of thearbitrary waveform generator signal at the other one of the firstultrasonic clamp-on transducer and the second ultrasonic clamp-ontransducer, deriving the measuring signal from the response signal, thederivation of the measuring signal comprising selecting a signal portionof the respective response signal or of a signal derived therefrom andreversing the signal portion with respect to time, storing the measuringsignal for later use.
 11. The method of claim 9, comprising repeatingthe steps of applying an arbitrary waveform generator signal andreceiving a corresponding response signal multiple times, therebyobtaining a plurality of response signals, deriving the respectivemeasuring signal from an average of the received response signals. 12.The method of claim 11, wherein the derivation of the respectivemeasuring signal comprises digitizing the corresponding response signalor a signal derived therefrom with respect to amplitude.
 13. The methodof claim 12, further comprising increasing the bit-resolution of thedigitized signal for increasing an amplitude of a response signal to therespective measuring signal.
 14. The method of claim 12, furthercomprising decreasing the bit-resolution of the digitized signal forincreasing an amplitude of a response signal to the respective measuringsignal.
 15. The method of claim 12, wherein the bit resolution of thedigitized signal with respect to the amplitude is a low bit resolution.16. A device for measuring a flow speed of a fluid in a conduit with atravel time ultrasonic flow meter, comprising a first connector forconnecting a first ultrasonic element a second connector for connectinga second ultrasonic element, a third connector for connecting a thirdultrasonic element, a transmitting unit for sending arbitrary waveformgenerator signals and for sending measuring signals, a receiving unitfor receiving response signals, a processing unit for deriving a firstmeasuring signal from a first inverted signal, for deriving a secondmeasuring signal from a second inverted signal and for storing the firstmeasuring signal and the second measuring signal, wherein the derivationof the first inverted signal and of the second inverted signal comprisesreversing a signal portion of a response signal of a correspondingarbitrary waveform generator signal or of a signal derived therefromwith respect to time, and wherein the processing unit, the transmittingunit and the receiving unit are operative to apply the first measuringsignal to the first connector, and to receive a first response signal ofthe first measuring signal at the second connector, to apply a secondmeasuring signal to the first connector, to receive a second responsesignal of the second measuring signal at the third connector and toderive a flow speed of the fluid from at least one of the first responsesignal and the second response signal.
 17. A device for measuring a flowspeed of a fluid in a fluid conduit with a travel time ultrasonic flowmeter, comprising a first connector, a first ultrasonic clamp-ontransducer which is connected to the first connector, a secondconnector, a second ultrasonic clamp-on transducer which is connected tothe second connector, a portion of the fluid conduit the firstultrasonic clamp-on transducer being mounted to the fluid conduitportion at a first location, and the second ultrasonic clamp-ontransducer being mounted to the conduit portion at a second location,wherein a connection line between the first ultrasonic clamp-ontransducer and the second clamp-on ultrasonic transducer extends outsideof a symmetry axis of the fluid conduit a transmitting unit for sendingarbitrary waveform generator signals and for sending measuring signals,a receiving unit for receiving response signals, a processing unit forderiving a measuring signal from an invalid signal, wherein thederivation of the inverted signal comprises reversing a signal portionof a response signal of a corresponding arbitrary waveform generatorsignal or of a signal derived therefrom with respect to time, andwherein the processing unit, the transmitting unit and the receivingunit are operative to apply the measuring signal to the first connector,to receive a response signal of the first measuring signal at the secondconnector, and to derive a flow speed of the fluid from the responsesignal.
 18. The device of claim 17, further comprising: a D/A converter,the D/A converter being connected to the first connector, an A/Dconverter, the A/D converter being connected to the second connector, acomputer readable memory for storing the measuring signal.
 19. Thedevice of claim 17, further comprising a selection unit, the selectionunit being operative to select a portion of a received response signalto the arbitrary waveform generator signal or a signal derivedtherefrom, and an inverting unit, the inverting unit being operative toinvert the selected portion of the received response signal with respectto time to obtain the inverted signal.
 20. The device of claim 17, thedevice comprising a direct digital signal synthesizer, the directdigital signal synthesizer comprising the A/D converter, a frequencycontrol register, a reference oscillator, a numerically controlledoscillator and a reconstruction low pass filter, the A/D converter beingconnectable to the first and the second connector over thereconstruction low pass filter.
 21. A method for determining whether atest device is measuring a flow speed of a fluid in a fluid conduitaccording to claim 1, comprising: providing the fluid conduit with afluid that has a pre-determined velocity with respect to the fluidconduit, providing the fluid conduit with a first ultrasonic transducerand a second ultrasonic transducer, applying a first test arbitrarywaveform generator signal to the first ultrasonic transducer of the testdevice, receiving a first test response signal of the first testarbitrary waveform generator signal at the second ultrasonic transducerof the test device, deriving a first test measuring signal from thefirst test response signal the derivation of the first test measuringsignal comprising reversing the first test response signal, or a portionthereof with respect to time, comparing the first test measuring signalwith a first measuring signal that is emitted at a transducer of thetest device, wherein it is determined that the test device is using amethod to determine a flow speed of a fluid in a fluid conduit accordingto claim 1, if the first test measuring signal and the first measuringsignal are similar.
 22. The method of claim 21, further comprisingproviding the fluid conduit with a third ultrasonic transducer, applyinga test arbitrary waveform generator signal to the first ultrasonictransducer of the test device or to the second ultrasonic transducer ofthe test device, receiving a second test response signal of the testarbitrary waveform generator signal at the third ultrasonic transducerof the test device, deriving a second test measuring signal from thesecond test response signal, comparing the second test measuring signalwith a second measuring signal that is emitted at a transducer of thetest device, wherein it is determined that the test device is using amethod to determine a flow speed of a fluid in a fluid conduit accordingto claim 1, if the first test measuring signal and the first measuringsignal are similar.
 23. A method for determining a flow speed of a fluidin a fluid conduit comprising: providing the fluid conduit with a fluidthat has a predetermined velocity with respect to the fluid conduit,providing the fluid conduit with a first ultrasonic wet transducer and asecond ultrasonic wet transducer, wherein a connection line between thefirst ultrasonic wet transducer and the second ultrasonic wet transducerextends outside of a symmetry axis of the fluid conduit, applying ameasuring signal to the first ultrasonic wet transducer, measuring aresponse signal of the measuring signal at the second ultrasonic wettransducer, wherein the measuring signal comprises a reversed signalportion with respect to time of a response signal of a correspondingimpulse signal or of a signal derived therefrom, deriving a flow speedof the fluid from the response signal.
 24. The method of claim 23,further comprising: applying a reverse direction measuring signal to thesecond ultrasonic wet transducer, measuring a reverse direction responsesignal of the measuring signal at the first ultrasonic wet transducer,wherein the measuring signal comprises a reversed signal portion withrespect to time of a response signal of a corresponding impulse signalor of a signal derived therefrom, deriving a flow speed of the fluidfrom the response signal.
 25. The method of claim 23, further comprisingprocessing of at least one of the response signals for determining achange in the wall thickness of the conduit or for determining materialcharacteristics of the conduit walls by determining longitudinal andtransversal sound wave characteristics.
 26. The method of claim 23,further comprising: providing the fluid conduit with a fluid, providingan impulse signal to one of the first ultrasonic wet transducer and thesecond ultrasonic wet transducer, receiving a response signal of theimpulse signal at the other one of the first ultrasonic wet transducerand the second ultrasonic wet transducer, deriving the measuring signalfrom the response signal, the derivation of the measuring signalcomprising selecting a signal portion of the respective response signalor of a signal derived therefrom and reversing the signal portion withrespect to time, storing the measuring signal for later use.
 27. Adevice for measuring a flow speed of a fluid in a fluid conduit with atravel time ultrasonic flow meter, comprising a first connector, a firstultrasonic wet transducer which is connected to the first connector, asecond connector, a second ultrasonic wet transducer which is connectedto the second connector, a portion of the fluid conduit the firstultrasonic wet transducer being mounted to the fluid conduit portion ata first location, and the second ultrasonic wet transducer being mountedto the conduit portion at a second location, wherein a connection linebetween the first ultrasonic wet transducer and the second ultrasonicwet transducer extends outside of a symmetry axis of the fluid conduit atransmitting unit for sending impulse signals and for sending measuringsignals, a receiving unit for receiving response signals, a processingunit for deriving a measuring signal from an inverted signal, whereinthe derivation of the inverted signal comprises reversing a signalportion of a response signal of a corresponding impulse signal or of asignal derived therefrom with respect to time, and wherein theprocessing unit, the transmitting unit and the receiving unit areoperative to apply the measuring signal to the first connector, toreceive a response signal of the first measuring signal at the secondconnector, and to derive a flow speed of the fluid from the responsesignal.
 28. The device of claim 27, further comprising: a D/A converter,the D/A converter being connected to the first connector, an A/Dconverter, the A/D converter being connected to the second connector, acomputer readable memory for storing the measuring signal.
 29. Thedevice of claim 27, further comprising a selection unit, the selectionunit being operative to select a portion of a received response signalto the impulse signal or a signal derived therefrom, and an invertingunit, the inverting unit being operative to invert the selected portionof the received response signal with respect to time to obtain theinverted signal.
 30. The device of claim 27, the device comprising: adirect digital signal synthesizer, the direct digital signal synthesizercomprising the A/D converter, a frequency control register, a referenceoscillator, a numerically controlled oscillator and a reconstruction lowpass filter, the A/D converter being connectable to the first and thesecond connector over the reconstruction low pass filter.